[swift-evolution] Strings in Swift 4

[Olivier Tardieu]

Thanks for the great write-up!

The manifesto recognizes the importance of machine processing and 
performance.
I am surprised that there is no mention of any kind of "unsafe" strings or 
string processing.
In general, Swift does an amazing job at incorporating unsafe mechanism 
into a safe-by-default programming paradigm.
But for some reason, Strings seem to be left out of the unsafe discussion.

A lot of machine processing of strings continues to deal with 8-bit 
quantities (even 7-bit quantities, not UTF-8).
Swift strings are not very good at that. I see progress in the manifesto 
but nothing to really close the performance gap with C.
That's where "unsafe" mechanisms could come into play.

To guarantee Unicode correctness, a C string must be validated or 
transformed to be considered a Swift string.
If I understand the C String interop section correctly, in Swift 4, this 
should not force a copy, but traversing the string is still required.
I hope I am correct about the no-copy thing, and I would also like to 
permit promoting C strings to Swift strings without validation.
This is obviously unsafe in general, but I know my strings... and I care 
about performance. ;)

More importantly, it is not possible to mutate bytes in a Swift string at 
will.
Again it makes sense from the point of view of always correct Unicode 
sequences.
But it does not for machine processing of C strings with C-like 
performance.
Today, I can cheat using a "_public" API for this, i.e., myString._core.
_baseAddress!.
This should be doable from an official "unsafe" API.

Memory safety is also at play here, as well as ownership.
A proper API could guarantee the backing store is writable for instance, 
that it is not shared.
A memory-safe but not unicode-safe API could do bounds checks.

While low-level C string processing can be done using unsafe memory 
buffers with performance, the lack of bridging with "real" Swift strings 
kills the deal.
No literals syntax (or costly coercions), none of the many useful string 
APIs.

To illustrate these points here is a simple experiment: code written to 
synthesize an http date string from a bunch of integers.
There are four versions of the code going from nice high-level Swift code 
to low-level C-like code.
(Some of this code is also about avoiding ARC overheads, and string 
interpolation overheads, hence the four versions.)

On my macbook pro (swiftc -O), the performance is as follows:

interpolation + func:  2.303032365s
interpolation + array: 1.224858418s
append:                0.918512377s
memcpy:                0.182104674s

While the benchmarking could be done more carefully, I think the main 
observation is valid.
The nice code is more than 10x slower than the C-like code.
Moreover, the ugly-but-still-valid-Swift code is still about 5x slower 
than the C like code.
For some applications, e.g. web servers, this kind of numbers matter...

Some of the proposed improvements would help with this, e.g., small 
strings optimization, and maybe changes to the concatenation semantics.
But it seems to me that a big performance gap will remain.
(Concatenation even with strncat is significantly slower than memcpy for 
fixed-size strings.)

I believe there is a need and an opportunity for a fast "less safe" String 
API.
I hope it will be on the roadmap soon.

Best,

Olivier

import Foundation

// get current date as a series of integers
// (could be done differently... faster... not the topic)

var theTime = time(nil)
var timeStruct = tm()
gmtime_r(&theTime, &timeStruct)
let wday = Int(timeStruct.tm_wday)
let mday = Int(timeStruct.tm_mday)
let mon = Int(timeStruct.tm_mon)
let year = Int(timeStruct.tm_year) + 1900
let hour = Int(timeStruct.tm_hour)
let min = Int(timeStruct.tm_min)
let sec = Int(timeStruct.tm_sec)

let months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun",
              "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]

let days = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]

func twoDigit(_ num: Int) -> String {
    return (num < 10 ? "0" : "") + String(num)
}

let twoDigit = ["00", "01", "02", "03", "04", "05", "06", "07", "08", "09"
,
                "10", "11", "12", "13", "14", "15", "16", "17", "18", "19"
,
                "20", "21", "22", "23", "24", "25", "26", "27", "28", "29"
,
                "30", "31", "32", "33", "34", "35", "36", "37", "38", "39"
,
                "40", "41", "42", "43", "44", "45", "46", "47", "48", "49"
,
                "50", "51", "52", "53", "54", "55", "56", "57", "58", "59"
,
                "60", "61", "62", "63", "64", "65", "66", "67", "68", "69"
,
                "70", "71", "72", "73", "74", "75", "76", "77", "78", "79"
,
                "80", "81", "82", "83", "84", "85", "86", "87", "88", "89"
,
                "90", "91", "92", "93", "94", "95", "96", "97", "98", "99"
]

// interpolation + func

func httpDate() -> String {
    return "\(days[wday]), \(twoDigit(mday)) \(months[mon]) \(year) \(
twoDigit(hour)):\(twoDigit(min)):\(twoDigit(sec)) GMT"
}

// interpolation + array

func httpDate1() -> String {
    return "\(days[wday]), \(twoDigit[mday]) \(months[mon]) \(year) \(
twoDigit[hour]):\(twoDigit[min]):\(twoDigit[sec]) GMT"
}

// append + array

func httpDate2() -> String {
    var s = days[wday]
    s.append(", ")
    s.append(twoDigit[mday])
    s.append(" ")
    s.append(months[mon])
    s.append(" ")
    s.append(twoDigit[year/100])
    s.append(twoDigit[year%100])
    s.append(" ")
    s.append(twoDigit[hour])
    s.append(":")
    s.append(twoDigit[min])
    s.append(":")
    s.append(twoDigit[sec])
    s.append(" GMT")
    return s
}

// memcpy + array

func httpDate3() -> String {
    var s = "XXX, XX XXX XXXX XX:XX:XX GMT"
    s.append("") // force alloc
    let ptr = s._core._baseAddress!
    memcpy(ptr, days[wday]._core._baseAddress!, 3)
    memcpy(ptr.advanced(by: 8), months[mon]._core._baseAddress!, 3)
    memcpy(ptr.advanced(by: 5), twoDigit[mday]._core._baseAddress!, 2)
    memcpy(ptr.advanced(by: 12), twoDigit[year/100]._core._baseAddress!, 2
)
    memcpy(ptr.advanced(by: 14), twoDigit[year%100]._core._baseAddress!, 2
)
    memcpy(ptr.advanced(by: 17), twoDigit[hour]._core._baseAddress!, 2)
    memcpy(ptr.advanced(by: 20), twoDigit[min]._core._baseAddress!, 2)
    memcpy(ptr.advanced(by: 23), twoDigit[sec]._core._baseAddress!, 2)
    return s
}

var s = ""

var now = mach_absolute_time()
for _ in 0..<1000000 {
    s = httpDate()
}
print(s)
print("interpolation + func: \(Double(mach_absolute_time() - now) / 1e9
)s\n")

now = mach_absolute_time()
for _ in 0..<1000000 {
    s = httpDate1()
}
print(s)
print("interpolation + array: \(Double(mach_absolute_time() - now) / 1e9
)s\n")

now = mach_absolute_time()
for _ in 0..<1000000 {
    s = httpDate2()
}
print(s)
print("append: \(Double(mach_absolute_time() - now) / 1e9)s\n")

now = mach_absolute_time()
for _ in 0..<1000000 {
    s = httpDate3()
}
print(s)
print("memcpy: \(Double(mach_absolute_time() - now) / 1e9)s\n")




From:   Ben Cohen via swift-evolution <swift-evolution at swift.org>
To:     swift-evolution <swift-evolution at swift.org>
Cc:     Dave Abrahams <dabrahams at apple.com>
Date:   01/19/2017 09:56 PM
Subject:        [swift-evolution] Strings in Swift 4
Sent by:        swift-evolution-bounces at swift.org



Hi all,

Below is our take on a design manifesto for Strings in Swift 4 and beyond.

Probably best read in rendered markdown on GitHub:
https://github.com/apple/swift/blob/master/docs/StringManifesto.md

We’re eager to hear everyone’s thoughts.

Regards,
Ben and Dave


# String Processing For Swift 4

* Authors: [Dave Abrahams](https://github.com/dabrahams), [Ben Cohen](
https://github.com/airspeedswift)

The goal of re-evaluating Strings for Swift 4 has been fairly ill-defined 
thus
far, with just this short blurb in the
[list of goals](
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025676.html
):

> **String re-evaluation**: String is one of the most important 
fundamental
> types in the language.  The standard library leads have numerous ideas 
of how
> to improve the programming model for it, without jeopardizing the goals 
of
> providing a unicode-correct-by-default model.  Our goal is to be better 
at
> string processing than Perl!
 
For Swift 4 and beyond we want to improve three dimensions of text 
processing:

  1. Ergonomics
  2. Correctness
  3. Performance

This document is meant to both provide a sense of the long-term vision 
(including undecided issues and possible approaches), and to define the 
scope of
work that could be done in the Swift 4 timeframe.

## General Principles

### Ergonomics

It's worth noting that ergonomics and correctness are 
mutually-reinforcing.  An
API that is easy to use—but incorrectly—cannot be considered an ergonomic
success.  Conversely, an API that's simply hard to use is also hard to use
correctly.  Acheiving optimal performance without compromising ergonomics 
or
correctness is a greater challenge.

Consistency with the Swift language and idioms is also important for
ergonomics. There are several places both in the standard library and in 
the
foundation additions to `String` where patterns and practices found 
elsewhere
could be applied to improve usability and familiarity.

### API Surface Area

Primary data types such as `String` should have APIs that are easily 
understood
given a signature and a one-line summary.  Today, `String` fails that 
test.  As
you can see, the Standard Library and Foundation both contribute 
significantly to
its overall complexity.

**Method Arity** | **Standard Library** | **Foundation**
---|:---:|:---:
0: `ƒ()` | 5 | 7
1: `ƒ(:)` | 19 | 48
2: `ƒ(::)` | 13 | 19
3: `ƒ(:::)` | 5 | 11
4: `ƒ(::::)` | 1 | 7
5: `ƒ(:::::)` | - | 2
6: `ƒ(::::::)` | - | 1

**API Kind** | **Standard Library** | **Foundation**
---|:---:|:---:
`init` | 41 | 18
`func` | 42 | 55
`subscript` | 9 | 0
`var` | 26 | 14

**Total: 205 APIs**

By contrast, `Int` has 80 APIs, none with more than two parameters.[0] 
String processing is complex enough; users shouldn't have
to press through physical API sprawl just to get started.

Many of the choices detailed below contribute to solving this problem,
including:

  * Restoring `Collection` conformance and dropping the `.characters` 
view.
  * Providing a more general, composable slicing syntax.
  * Altering `Comparable` so that parameterized
    (e.g. case-insensitive) comparison fits smoothly into the basic 
syntax.
  * Clearly separating language-dependent operations on text produced 
    by and for humans from language-independent
    operations on text produced by and for machine processing.
  * Relocating APIs that fall outside the domain of basic string 
processing and
    discouraging the proliferation of ad-hoc extensions.


### Batteries Included

While `String` is available to all programs out-of-the-box, crucial APIs 
for
basic string processing tasks are still inaccessible until `Foundation` is
imported.  While it makes sense that `Foundation` is needed for 
domain-specific
jobs such as
[linguistic tagging](
https://developer.apple.com/reference/foundation/nslinguistictagger),
one should not need to import anything to, for example, do 
case-insensitive
comparison.

### Unicode Compliance and Platform Support

The Unicode standard provides a crucial objective reference point for what
constitutes correct behavior in an extremely complex domain, so
Unicode-correctness is, and will remain, a fundamental design principle 
behind
Swift's `String`.  That said, the Unicode standard is an evolving 
document, so
this objective reference-point is not fixed.[1] While
many of the most important operations—e.g. string hashing, equality, and
non-localized comparison—will be stable, the semantics
of others, such as grapheme breaking and localized comparison and case
conversion, are expected to change as platforms are updated, so programs 
should
be written so their correctness does not depend on precise stability of 
these
semantics across OS versions or platforms.  Although it may be possible to
imagine static and/or dynamic analysis tools that will help users find 
such
errors, the only sure way to deal with this fact of life is to educate 
users.

## Design Points

### Internationalization

There is strong evidence that developers cannot determine how to use
internationalization APIs correctly.  Although documentation could and 
should be
improved, the sheer size, complexity, and diversity of these APIs is a 
major
contributor to the problem, causing novices to tune out, and more 
experienced
programmers to make avoidable mistakes.

The first step in improving this situation is to regularize all localized
operations as invocations of normal string operations with extra
parameters. Among other things, this means:

1. Doing away with `localizedXXX` methods 
2. Providing a terse way to name the current locale as a parameter
3. Automatically adjusting defaults for options such
   as case sensitivity based on whether the operation is localized.
4. Removing correctness traps like `localizedCaseInsensitiveCompare` (see
    guidance in the
    [Internationalization and Localization Guide](
https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/InternationalizingYourCode/InternationalizingYourCode.html
).

Along with appropriate documentation updates, these changes will make 
localized
operations more teachable, comprehensible, and approachable, thereby 
lowering a
barrier that currently leads some developers to ignore localization issues
altogether.

####  The Default Behavior of `String`

Although this isn't well-known, the most accessible form of many 
operations on
Swift `String` (and `NSString`) are really only appropriate for text that 
is
intended to be processed for, and consumed by, machines.  The semantics of 
the
operations with the simplest spellings are always non-localized and
language-agnostic.

Two major factors play into this design choice:

1. Machine processing of text is important, so we should have first-class,
   accessible functions appropriate to that use case.
 
2. The most general localized operations require a locale parameter not 
required
   by their un-localized counterparts.  This naturally skews complexity 
towards
   localized operations.

Reaffirming that `String`'s simplest APIs have
language-independent/machine-processed semantics has the benefit of 
clarifying
the proper default behavior of operations such as comparison, and allows 
us to
make [significant optimizations](#collation-semantics) that were 
previously
thought to conflict with Unicode.

#### Future Directions

One of the most common internationalization errors is the unintentional
presentation to users of text that has not been localized, but 
regularizing APIs
and improving documentation can go only so far in preventing this error.
Combined with the fact that `String` operations are non-localized by 
default,
the environment for processing human-readable text may still be somewhat
error-prone in Swift 4.

For an audience of mostly non-experts, it is especially important that 
naïve
code is very likely to be correct if it compiles, and that more 
sophisticated
issues can be revealed progressively.  For this reason, we intend to
specifically and separately target localization and internationalization
problems in the Swift 5 timeframe.

### Operations With Options

There are three categories of common string operation that commonly need 
to be
tuned in various dimensions:

**Operation**|**Applicable Options**
---|---
sort ordering | locale, case/diacritic/width-insensitivity
case conversion | locale
pattern matching | locale, case/diacritic/width-insensitivity

The defaults for case-, diacritic-, and width-insensitivity are different 
for
localized operations than for non-localized operations, so for example a
localized sort should be case-insensitive by default, and a non-localized 
sort
should be case-sensitive by default.  We propose a standard “language” of
defaulted parameters to be used for these purposes, with usage roughly 
like this:

```swift
  x.compared(to: y, case: .sensitive, in: swissGerman)
 
  x.lowercased(in: .currentLocale)
 
  x.allMatches(
    somePattern, case: .insensitive, diacritic: .insensitive)
```

This usage might be supported by code like this:

```swift
enum StringSensitivity {
case sensitive
case insensitive
}

extension Locale {
  static var currentLocale: Locale { ... }
}

extension Unicode {
  // An example of the option language in declaration context,
  // with nil defaults indicating unspecified, so defaults can be
  // driven by the presence/absence of a specific Locale
  func frobnicated(
    case caseSensitivity: StringSensitivity? = nil,
    diacritic diacriticSensitivity: StringSensitivity? = nil,
    width widthSensitivity: StringSensitivity? = nil,
    in locale: Locale? = nil
  ) -> Self { ... }
}
```

### Comparing and Hashing Strings

#### Collation Semantics

What Unicode says about collation—which is used in `<`, `==`, and hashing— 
turns
out to be quite interesting, once you pick it apart.  The full Unicode 
Collation
Algorithm (UCA) works like this:

1. Fully normalize both strings
2. Convert each string to a sequence of numeric triples to form a 
collation key
3. “Flatten” the key by concatenating the sequence of first elements to 
the
   sequence of second elements to the sequence of third elements
4. Lexicographically compare the flattened keys 

While step 1 can usually
be [done quickly](http://unicode.org/reports/tr15/#Description_Norm) and
incrementally, step 2 uses a collation table that maps matching 
*sequences* of
unicode scalars in the normalized string to *sequences* of triples, which 
get
accumulated into a collation key.  Predictably, this is where the real 
costs
lie.

*However*, there are some bright spots to this story.  First, as it turns 
out,
string sorting (localized or not) should be done down to what's called
the
[“identical” level](
http://unicode.org/reports/tr10/#Multi_Level_Comparison),
which adds a step 3a: append the string's normalized form to the flattened
collation key.  At first blush this just adds work, but consider what it 
does
for equality: two strings that normalize the same, naturally, will collate 
the
same.  But also, *strings that normalize differently will always collate
differently*.  In other words, for equality, it is sufficient to compare 
the
strings' normalized forms and see if they are the same.  We can therefore
entirely skip the expensive part of collation for equality comparison.

Next, naturally, anything that applies to equality also applies to 
hashing: it
is sufficient to hash the string's normalized form, bypassing collation 
keys.
This should provide significant speedups over the current implementation.
Perhaps more importantly, since comparison down to the “identical” level 
applies
even to localized strings, it means that hashing and equality can be 
implemented
exactly the same way for localized and non-localized text, and hash tables 
with
localized keys will remain valid across current-locale changes.

Finally, once it is agreed that the *default* role for `String` is to 
handle
machine-generated and machine-readable text, the default ordering of 
`String`s
need no longer use the UCA at all.  It is sufficient to order them in any 
way
that's consistent with equality, so `String` ordering can simply be a
lexicographical comparison of normalized forms,[4]
(which is equivalent to lexicographically comparing the sequences of 
grapheme
clusters), again bypassing step 2 and offering another speedup.

This leaves us executing the full UCA *only* for localized sorting, and 
ICU's
implementation has apparently been very well optimized.

Following this scheme everywhere would also allow us to make sorting 
behavior
consistent across platforms.  Currently, we sort `String` according to the 
UCA,
except that—*only on Apple platforms*—pairs of ASCII characters are 
ordered by
unicode scalar value.

#### Syntax

Because the current `Comparable` protocol expresses all comparisons with 
binary
operators, string comparisons—which may require
additional [options](#operations-with-options)—do not fit smoothly into 
the
existing syntax.  At the same time, we'd like to solve other problems with
comparison, as outlined
in
[this proposal](
https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e)
(implemented by changes at the head
of
[this branch](
https://github.com/CodaFi/swift/commits/space-the-final-frontier)).
We should adopt a modification of that proposal that uses a method rather 
than
an operator `<=>`:

```swift
enum SortOrder { case before, same, after }

protocol Comparable : Equatable {
 func compared(to: Self) -> SortOrder
 ...
}
```

This change will give us a syntactic platform on which to implement 
methods with
additional, defaulted arguments, thereby unifying and regularizing 
comparison
across the library.

```swift
extension String {
 func compared(to: Self) -> SortOrder

}
```

**Note:** `SortOrder` should bridge to `NSComparisonResult`.  It's also 
possible
that the standard library simply adopts Foundation's `ComparisonResult` as 
is,
but we believe the community should at least consider alternate naming 
before
that happens.  There will be an opportunity to discuss the choices in 
detail
when the modified
[Comparison Proposal](
https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e) comes
up for review.

### `String` should be a `Collection` of `Character`s Again

In Swift 2.0, `String`'s `Collection` conformance was dropped, because we
convinced ourselves that its semantics differed from those of `Collection` 
too
significantly.

It was always well understood that if strings were treated as sequences of
`UnicodeScalar`s, algorithms such as `lexicographicalCompare`, 
`elementsEqual`,
and `reversed` would produce nonsense results. Thus, in Swift 1.0, 
`String` was
a collection of `Character` (extended grapheme clusters). During 2.0
development, though, we realized that correct string concatenation could
occasionally merge distinct grapheme clusters at the start and end of 
combined
strings.

This quirk aside, every aspect of strings-as-collections-of-graphemes 
appears to
comport perfectly with Unicode. We think the concatenation problem is 
tolerable,
because the cases where it occurs all represent partially-formed 
constructs. The
largest class—isolated combining characters such as ◌́ (U+0301 COMBINING 
ACUTE
ACCENT)—are explicitly called out in the Unicode standard as
“[degenerate](http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries
)” or
“[defective](http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf)”. The 
other
cases—such as a string ending in a zero-width joiner or half of a regional
indicator—appear to be equally transient and unlikely outside of a text 
editor.

Admitting these cases encourages exploration of grapheme composition and 
is
consistent with what appears to be an overall Unicode philosophy that “no
special provisions are made to get marginally better behavior for… cases 
that
never occur in practice.”[2] Furthermore, it seems
unlikely to disturb the semantics of any plausible algorithms. We can 
handle
these cases by documenting them, explicitly stating that the elements of a
`String` are an emergent property based on Unicode rules.

The benefits of restoring `Collection` conformance are substantial: 

  * Collection-like operations encourage experimentation with strings to
    investigate and understand their behavior. This is useful for teaching 
new
    programmers, but also good for experienced programmers who want to
    understand more about strings/unicode.
 
  * Extended grapheme clusters form a natural element boundary for Unicode
    strings.  For example, searching and matching operations will always 
produce
    results that line up on grapheme cluster boundaries.
 
  * Character-by-character processing is a legitimate thing to do in many 
real
    use-cases, including parsing, pattern matching, and language-specific
    transformations such as transliteration.
 
  * `Collection` conformance makes a wide variety of powerful operations
    available that are appropriate to `String`'s default role as the 
vehicle for
    machine processed text.
 
    The methods `String` would inherit from `Collection`, where similar to
    higher-level string algorithms, have the right semantics.  For 
example,
    grapheme-wise `lexicographicalCompare`, `elementsEqual`, and 
application of
    `flatMap` with case-conversion, produce the same results one would 
expect
    from whole-string ordering comparison, equality comparison, and
    case-conversion, respectively.  `reverse` operates correctly on 
graphemes,
    keeping diacritics moored to their base characters and leaving emoji 
intact.
    Other methods such as `indexOf` and `contains` make obvious sense. A 
few
    `Collection` methods, like `min` and `max`, may not be particularly 
useful
    on `String`, but we don't consider that to be a problem worth solving, 
in
    the same way that we wouldn't try to suppress `min` and `max` on a
    `Set([UInt8])` that was used to store IP addresses.
 
  * Many of the higher-level operations that we want to provide for 
`String`s,
    such as parsing and pattern matching, should apply to any 
`Collection`, and
    many of the benefits we want for `Collections`, such
    as unified slicing, should accrue
    equally to `String`.  Making `String` part of the same protocol 
hierarchy
    allows us to write these operations once and not worry about keeping 
the
    benefits in sync.
 
  * Slicing strings into substrings is a crucial part of the vocabulary of
    string processing, and all other sliceable things are `Collection`s.
    Because of its collection-like behavior, users naturally think of 
`String`
    in collection terms, but run into frustrating limitations where it 
fails to
    conform and are left to wonder where all the differences lie.  Many 
simply
    “correct” this limitation by declaring a trivial conformance:
 
    ```swift
  extension String : BidirectionalCollection {}
    ```
 
    Even if we removed indexing-by-element from `String`, users could 
still do
    this:
 
    ```swift
      extension String : BidirectionalCollection {
        subscript(i: Index) -> Character { return characters[i] }
      }
    ```
 
    It would be much better to legitimize the conformance to `Collection` 
and
    simply document the oddity of any concatenation corner-cases, than to 
deny
    users the benefits on the grounds that a few cases are confusing.

Note that the fact that `String` is a collection of graphemes does *not* 
mean
that string operations will necessarily have to do grapheme boundary
recognition.  See the Unicode protocol section for details.

### `Character` and `CharacterSet`

`Character`, which represents a
Unicode
[extended grapheme cluster](
http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries),
is a bit of a black box, requiring conversion to `String` in order to
do any introspection, including interoperation with ASCII.  To fix this, 
we should:

 - Add a `unicodeScalars` view much like `String`'s, so that the 
sub-structure
   of grapheme clusters is discoverable.
 - Add a failable `init` from sequences of scalars (returning nil for 
sequences
   that contain 0 or 2+ graphemes).
 - (Lower priority) expose some operations, such as `func uppercase() ->
   String`, `var isASCII: Bool`, and, to the extent they can be sensibly
   generalized, queries of unicode properties that should also be exposed 
on
   `UnicodeScalar` such as `isAlphabetic` and `isGraphemeBase` .

Despite its name, `CharacterSet` currently operates on the Swift 
`UnicodeScalar`
type. This means it is usable on `String`, but only by going through the 
unicode
scalar view. To deal with this clash in the short term, `CharacterSet` 
should be
renamed to `UnicodeScalarSet`.  In the longer term, it may be appropriate 
to
introduce a `CharacterSet` that provides similar functionality for 
extended
grapheme clusters.[5]

### Unification of Slicing Operations

Creating substrings is a basic part of String processing, but the slicing
operations that we have in Swift are inconsistent in both their spelling 
and
their naming: 

  * Slices with two explicit endpoints are done with subscript, and 
support
    in-place mutation:
 
    ```swift
        s[i..<j].mutate()
    ```

  * Slicing from an index to the end, or from the start to an index, is 
done
    with a method and does not support in-place mutation:
    ```swift
        s.prefix(upTo: i).readOnly()
    ```

Prefix and suffix operations should be migrated to be subscripting 
operations
with one-sided ranges i.e. `s.prefix(upTo: i)` should become `s[..<i]`, as
in
[this proposal](
https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0132-sequence-end-ops.md
).
With generic subscripting in the language, that will allow us to collapse 
a wide
variety of methods and subscript overloads into a single implementation, 
and
give users an easy-to-use and composable way to describe subranges.

Further extending this EDSL to integrate use-cases like 
`s.prefix(maxLength: 5)`
is an ongoing research project that can be considered part of the 
potential
long-term vision of text (and collection) processing.

### Substrings

When implementing substring slicing, languages are faced with three 
options:

1. Make the substrings the same type as string, and share storage.
2. Make the substrings the same type as string, and copy storage when 
making the substring.
3. Make substrings a different type, with a storage copy on conversion to 
string.

We think number 3 is the best choice. A walk-through of the tradeoffs 
follows.

#### Same type, shared storage

In Swift 3.0, slicing a `String` produces a new `String` that is a view 
into a
subrange of the original `String`'s storage. This is why `String` is 3 
words in
size (the start, length and buffer owner), unlike the similar `Array` type
which is only one.

This is a simple model with big efficiency gains when chopping up strings 
into
multiple smaller strings. But it does mean that a stored substring keeps 
the
entire original string buffer alive even after it would normally have been
released.

This arrangement has proven to be problematic in other programming 
languages,
because applications sometimes extract small strings from large ones and 
keep
those small strings long-term. That is considered a memory leak and was 
enough
of a problem in Java that they changed from substrings sharing storage to
making a copy in 1.7.

#### Same type, copied storage

Copying of substrings is also the choice made in C#, and in the default
`NSString` implementation. This approach avoids the memory leak issue, but 
has
obvious performance overhead in performing the copies.

This in turn encourages trafficking in string/range pairs instead of in
substrings, for performance reasons, leading to API challenges. For 
example:

```swift
foo.compare(bar, range: start..<end)
```

Here, it is not clear whether `range` applies to `foo` or `bar`. This
relationship is better expressed in Swift as a slicing operation:

```swift
foo[start..<end].compare(bar)
```

Not only does this clarify to which string the range applies, it also 
brings
this sub-range capability to any API that operates on `String` "for free". 
So
these other combinations also work equally well:

```swift
// apply range on argument rather than target
foo.compare(bar[start..<end])
// apply range on both
foo[start..<end].compare(bar[start1..<end1])
// compare two strings ignoring first character
foo.dropFirst().compare(bar.dropFirst())
```

In all three cases, an explicit range argument need not appear on the 
`compare`
method itself. The implementation of `compare` does not need to know 
anything
about ranges. Methods need only take range arguments when that was an
integral part of their purpose (for example, setting the start and end of 
a
user's current selection in a text box).

#### Different type, shared storage

The desire to share underlying storage while preventing accidental memory 
leaks
occurs with slices of `Array`. For this reason we have an `ArraySlice` 
type.
The inconvenience of a separate type is mitigated by most operations used 
on
`Array` from the standard library being generic over `Sequence` or 
`Collection`.

We should apply the same approach for `String` by introducing a distinct
`SubSequence` type, `Substring`. Similar advice given for `ArraySlice` 
would apply to `Substring`:

> Important: Long-term storage of `Substring` instances is discouraged. A
> substring holds a reference to the entire storage of a larger string, 
not
> just to the portion it presents, even after the original string's 
lifetime
> ends. Long-term storage of a `Substring` may therefore prolong the 
lifetime
> of large strings that are no longer otherwise accessible, which can 
appear
> to be memory leakage.

When assigning a `Substring` to a longer-lived variable (usually a stored
property) explicitly of type `String`, a type conversion will be 
performed, and
at this point the substring buffer is copied and the original string's 
storage
can be released.

A `String` that was not its own `Substring` could be one word—a single 
tagged
pointer—without requiring additional allocations. `Substring`s would be a 
view
onto a `String`, so are 3 words - pointer to owner, pointer to start, and 
a
length. The small string optimization for `Substring` would take advantage 
of
the larger size, probably with a less compressed encoding for speed.

The downside of having two types is the inconvenience of sometimes having 
a
`Substring` when you need a `String`, and vice-versa. It is likely this 
would
be a significantly bigger problem than with `Array` and `ArraySlice`, as
slicing of `String` is such a common operation. It is especially relevant 
to
existing code that assumes `String` is the currency type. To ease the pain 
of
type mismatches, `Substring` should be a subtype of `String` in the same 
way
that `Int` is a subtype of `Optional<Int>`. This would give users an 
implicit
conversion from `Substring` to `String`, as well as the usual implicit
conversions such as `[Substring]` to `[String]` that other subtype
relationships receive.

In most cases, type inference combined with the subtype relationship 
should
make the type difference a non-issue and users will not care which type 
they
are using. For flexibility and optimizability, most operations from the
standard library will traffic in generic models of
[`Unicode`](#the--code-unicode--code--protocol).

##### Guidance for API Designers

In this model, **if a user is unsure about which type to use, `String` is 
always
a reasonable default**. A `Substring` passed where `String` is expected 
will be
implicitly copied. When compared to the “same type, copied storage” model, 
we
have effectively deferred the cost of copying from the point where a 
substring
is created until it must be converted to `String` for use with an API.

A user who needs to optimize away copies altogether should use this 
guideline:
if for performance reasons you are tempted to add a `Range` argument to 
your
method as well as a `String` to avoid unnecessary copies, you should 
instead
use `Substring`.

##### The “Empty Subscript”

To make it easy to call such an optimized API when you only have a 
`String` (or
to call any API that takes a `Collection`'s `SubSequence` when all you 
have is
the `Collection`), we propose the following “empty subscript” operation,

```swift
extension Collection {
  subscript() -> SubSequence { 
    return self[startIndex..<endIndex] 
  }
}
```

which allows the following usage:

```swift
funcThatIsJustLooking(at: person.name[]) // pass person.name as Substring
```

The `[]` syntax can be offered as a fixit when needed, similar to `&` for 
an
`inout` argument. While it doesn't help a user to convert `[String]` to
`[Substring]`, the need for such conversions is extremely rare, can be 
done with
a simple `map` (which could also be offered by a fixit):

```swift
takesAnArrayOfSubstring(arrayOfString.map { $0[] })
```

#### Other Options Considered

As we have seen, all three options above have downsides, but it's possible
these downsides could be eliminated/mitigated by the compiler. We are 
proposing
one such mitigation—implicit conversion—as part of the the "different 
type,
shared storage" option, to help avoid the cognitive load on developers of
having to deal with a separate `Substring` type.

To avoid the memory leak issues of a "same type, shared storage" substring
option, we considered whether the compiler could perform an implicit copy 
of
the underlying storage when it detects the string is being "stored" for 
long
term usage, say when it is assigned to a stored property. The trouble with 
this
approach is it is very difficult for the compiler to distinguish between
long-term storage versus short-term in the case of abstractions that rely 
on
stored properties. For example, should the storing of a substring inside 
an
`Optional` be considered long-term? Or the storing of multiple substrings
inside an array? The latter would not work well in the case of a
`components(separatedBy:)` implementation that intended to return an array 
of
substrings. It would also be difficult to distinguish intentional 
medium-term
storage of substrings, say by a lexer. There does not appear to be an 
effective
consistent rule that could be applied in the general case for detecting 
when a
substring is truly being stored long-term.

To avoid the cost of copying substrings under "same type, copied storage", 
the
optimizer could be enhanced to to reduce the impact of some of those 
copies.
For example, this code could be optimized to pull the invariant substring 
out
of the loop:

```swift
for _ in 0..<lots { 
  someFunc(takingString: bigString[bigRange]) 
}
```

It's worth noting that a similar optimization is needed to avoid an 
equivalent
problem with implicit conversion in the "different type, shared storage" 
case:

```swift
let substring = bigString[bigRange]
for _ in 0..<lots { someFunc(takingString: substring) }
```

However, in the case of "same type, copied storage" there are many use 
cases
that cannot be optimized as easily. Consider the following simple 
definition of
a recursive `contains` algorithm, which when substring slicing is linear 
makes
the overall algorithm quadratic:

```swift
extension String {
    func containsChar(_ x: Character) -> Bool {
        return !isEmpty && (first == x || dropFirst().containsChar(x))
    }
}
```

For the optimizer to eliminate this problem is unrealistic, forcing the 
user to
remember to optimize the code to not use string slicing if they want it to 
be
efficient (assuming they remember):

```swift
extension String {
    // add optional argument tracking progress through the string
    func containsCharacter(_ x: Character, atOrAfter idx: Index? = nil) -> 
Bool {
        let idx = idx ?? startIndex
        return idx != endIndex
            && (self[idx] == x || containsCharacter(x, atOrAfter: 
index(after: idx)))
    }
}
```

#### Substrings, Ranges and Objective-C Interop

The pattern of passing a string/range pair is common in several 
Objective-C
APIs, and is made especially awkward in Swift by the 
non-interchangeability of
`Range<String.Index>` and `NSRange`. 

```swift
s2.find(s2, sourceRange: NSRange(j..<s2.endIndex, in: s2))
```

In general, however, the Swift idiom for operating on a sub-range of a
`Collection` is to *slice* the collection and operate on that:

```swift
s2.find(s2[j..<s2.endIndex])
```

Therefore, APIs that operate on an `NSString`/`NSRange` pair should be 
imported
without the `NSRange` argument.  The Objective-C importer should be 
changed to
give these APIs special treatment so that when a `Substring` is passed, 
instead
of being converted to a `String`, the full `NSString` and range are passed 
to
the Objective-C method, thereby avoiding a copy.

As a result, you would never need to pass an `NSRange` to these APIs, 
which
solves the impedance problem by eliminating the argument, resulting in 
more
idiomatic Swift code while retaining the performance benefit.  To help 
users
manually handle any cases that remain, Foundation should be augmented to 
allow
the following syntax for converting to and from `NSRange`:

```swift
let nsr = NSRange(i..<j, in: s) // An NSRange corresponding to s[i..<j]
let iToJ = Range(nsr, in: s)    // Equivalent to i..<j
```

### The `Unicode` protocol

With `Substring` and `String` being distinct types and sharing almost all
interface and semantics, and with the highest-performance string 
processing
requiring knowledge of encoding and layout that the currency types can't
provide, it becomes important to capture the common “string API” in a 
protocol.
Since Unicode conformance is a key feature of string processing in swift, 
we
call that protocol `Unicode`:

**Note:** The following assumes several features that are planned but not 
yet implemented in
  Swift, and should be considered a sketch rather than a final design.
 
```swift
protocol Unicode 
  : Comparable, BidirectionalCollection where Element == Character {
 
  associatedtype Encoding : UnicodeEncoding
  var encoding: Encoding { get }
 
  associatedtype CodeUnits 
    : RandomAccessCollection where Element == Encoding.CodeUnit
  var codeUnits: CodeUnits { get }
 
  associatedtype UnicodeScalars 
    : BidirectionalCollection  where Element == UnicodeScalar
  var unicodeScalars: UnicodeScalars { get }

  associatedtype ExtendedASCII 
    : BidirectionalCollection where Element == UInt32
  var extendedASCII: ExtendedASCII { get }

  var unicodeScalars: UnicodeScalars { get }
}

extension Unicode {
  // ... define high-level non-mutating string operations, e.g. search ...

  func compared<Other: Unicode>(
    to rhs: Other,
    case caseSensitivity: StringSensitivity? = nil,
    diacritic diacriticSensitivity: StringSensitivity? = nil,
    width widthSensitivity: StringSensitivity? = nil,
    in locale: Locale? = nil
  ) -> SortOrder { ... }
}

extension Unicode : RangeReplaceableCollection where CodeUnits :
  RangeReplaceableCollection {
    // Satisfy protocol requirement
    mutating func replaceSubrange<C : Collection>(_: Range<Index>, with: 
C) 
      where C.Element == Element
 
  // ... define high-level mutating string operations, e.g. replace ...
}

```

The goal is that `Unicode` exposes the underlying encoding and code units 
in
such a way that for types with a known representation (e.g. a 
high-performance
`UTF8String`) that information can be known at compile-time and can be 
used to
generate a single path, while still allowing types like `String` that 
admit
multiple representations to use runtime queries and branches to fast path
specializations.

**Note:** `Unicode` would make a fantastic namespace for much of
what's in this proposal if we could get the ability to nest types and
protocols in protocols.


### Scanning, Matching, and Tokenization

#### Low-Level Textual Analysis

We should provide convenient APIs processing strings by character.  For 
example,
it should be easy to cleanly express, “if this string starts with `"f"`, 
process
the rest of the string as follows…”  Swift is well-suited to expressing 
this
common pattern beautifully, but we need to add the APIs.  Here are two 
examples
of the sort of code that might be possible given such APIs:

```swift
if let firstLetter = input.droppingPrefix(alphabeticCharacter) {
  somethingWith(input) // process the rest of input
}

if let (number, restOfInput) = input.parsingPrefix(Int.self) {
   ...
}
```

The specific spelling and functionality of APIs like this are TBD.  The 
larger
point is to make sure matching-and-consuming jobs are well-supported.

#### Unified Pattern Matcher Protocol

Many of the current methods that do matching are overloaded to do the same
logical operations in different ways, with the following axes:

- Logical Operation: `find`, `split`, `replace`, match at start
- Kind of pattern: `CharacterSet`, `String`, a regex, a closure
- Options, e.g. case/diacritic sensitivity, locale.  Sometimes a part of
  the method name, and sometimes an argument
- Whole string or subrange.

We should represent these aspects as orthogonal, composable components,
abstracting pattern matchers into a protocol like
[this one](
https://github.com/apple/swift/blob/master/test/Prototypes/PatternMatching.swift#L33
),
that can allow us to define logical operations once, without introducing
overloads, and massively reducing API surface area.

For example, using the strawman prefix `%` syntax to turn string literals 
into
patterns, the following pairs would all invoke the same generic methods:

```swift
if let found = s.firstMatch(%"searchString") { ... }
if let found = s.firstMatch(someRegex) { ... }

for m in s.allMatches((%"searchString"), case: .insensitive) { ... }
for m in s.allMatches(someRegex) { ... }

let items = s.split(separatedBy: ", ")
let tokens = s.split(separatedBy: CharacterSet.whitespace)
```

Note that, because Swift requires the indices of a slice to match the 
indices of
the range from which it was sliced, operations like `firstMatch` can 
return a
`Substring?` in lieu of a `Range<String.Index>?`: the indices of the match 
in
the string being searched, if needed, can easily be recovered as the
`startIndex` and `endIndex` of the `Substring`.

Note also that matching operations are useful for collections in general, 
and
would fall out of this proposal:

```
// replace subsequences of contiguous NaNs with zero
forces.replace(oneOrMore([Float.nan]), [0.0])
```

#### Regular Expressions

Addressing regular expressions is out of scope for this proposal.
That said, it is important that to note the pattern matching protocol 
mentioned
above provides a suitable foundation for regular expressions, and types 
such as
`NSRegularExpression` can easily be retrofitted to conform to it.  In the
future, support for regular expression literals in the compiler could 
allow for
compile-time syntax checking and optimization.

### String Indices

`String` currently has four views—`characters`, `unicodeScalars`, `utf8`, 
and
`utf16`—each with its own opaque index type.  The APIs used to translate 
indices
between views add needless complexity, and the opacity of indices makes 
them
difficult to serialize.

The index translation problem has two aspects:

  1. `String` views cannot consume one anothers' indices without a 
cumbersome
    conversion step.  An index into a `String`'s `characters` must be 
translated
    before it can be used as a position in its `unicodeScalars`.  Although 
these
    translations are rarely needed, they add conceptual and API 
complexity.
  2. Many APIs in the core libraries and other frameworks still expose 
`String`
    positions as `Int`s and regions as `NSRange`s, which can only 
reference a
    `utf16` view and interoperate poorly with `String` itself.

#### Index Interchange Among Views

String's need for flexible backing storage and reasonably-efficient 
indexing
(i.e. without dynamically allocating and reference-counting the indices
themselves) means indices need an efficient underlying storage type. 
Although
we do not wish to expose `String`'s indices *as* integers, `Int` offsets 
into
underlying code unit storage makes a good underlying storage type, 
provided
`String`'s underlying storage supports random-access.  We think 
random-access
*code-unit storage* is a reasonable requirement to impose on all `String`
instances.

Making these `Int` code unit offsets conveniently accessible and 
constructible
solves the serialization problem:

```swift
clipboard.write(s.endIndex.codeUnitOffset)
let offset = clipboard.read(Int.self)
let i = String.Index(codeUnitOffset: offset)
```

Index interchange between `String` and its `unicodeScalars`, `codeUnits`,
and [`extendedASCII`](#parsing-ascii-structure) views can be made entirely
seamless by having them share an index type (semantics of indexing a 
`String`
between grapheme cluster boundaries are TBD—it can either trap or be 
forgiving).
Having a common index allows easy traversal into the interior of 
graphemes,
something that is often needed, without making it likely that someone will 
do it
by accident.

 - `String.index(after:)` should advance to the next grapheme, even when 
the
   index points partway through a grapheme.
 
 - `String.index(before:)` should move to the start of the grapheme before
   the current position.

Seamless index interchange between `String` and its UTF-8 or UTF-16 views 
is not
crucial, as the specifics of encoding should not be a concern for most use
cases, and would impose needless costs on the indices of other views. That
said, we can make translation much more straightforward by exposing simple
bidirectional converting `init`s on both index types:

```swift
let u8Position = String.UTF8.Index(someStringIndex)
let originalPosition = String.Index(u8Position)
```

#### Index Interchange with Cocoa

We intend to address `NSRange`s that denote substrings in Cocoa APIs as
described [later in this 
document](#substrings--ranges-and-objective-c-interop).
That leaves the interchange of bare indices with Cocoa APIs trafficking in
`Int`.  Hopefully such APIs will be rare, but when needed, the following
extension, which would be useful for all `Collections`, can help:

```swift
extension Collection {
  func index(offset: IndexDistance) -> Index {
    return index(startIndex, offsetBy: offset)
  }
  func offset(of i: Index) -> IndexDistance {
    return distance(from: startIndex, to: i)
  }
}
```

Then integers can easily be translated into offsets into a `String`'s 
`utf16`
view for consumption by Cocoa:

```swift
let cocoaIndex = s.utf16.offset(of: String.UTF16Index(i))
let swiftIndex = s.utf16.index(offset: cocoaIndex)
```

### Formatting

A full treatment of formatting is out of scope of this proposal, but
we believe it's crucial for completing the text processing picture.  This
section details some of the existing issues and thinking that may guide 
future
development.

#### Printf-Style Formatting

`String.format` is designed on the `printf` model: it takes a format 
string with
textual placeholders for substitution, and an arbitrary list of other 
arguments.
The syntax and meaning of these placeholders has a long history in
C, but for anyone who doesn't use them regularly they are cryptic and 
complex,
as the `printf (3)` man page attests.

Aside from complexity, this style of API has two major problems: First, 
the
spelling of these placeholders must match up to the types of the 
arguments, in
the right order, or the behavior is undefined.  Some limited support for
compile-time checking of this correspondence could be implemented, but 
only for
the cases where the format string is a literal. Second, there's no 
reasonable
way to extend the formatting vocabulary to cover the needs of new types: 
you are
stuck with what's in the box.

#### Foundation Formatters

The formatters supplied by Foundation are highly capable and versatile, 
offering
both formatting and parsing services.  When used for formatting, though, 
the
design pattern demands more from users than it should:

  * Matching the type of data being formatted to a formatter type
  * Creating an instance of that type
  * Setting stateful options (`currency`, `dateStyle`) on the type.  Note: 
the
    need for this step prevents the instance from being used and discarded 
in
    the same expression where it is created.
  * Overall, introduction of needless verbosity into source

These may seem like small issues, but the experience of Apple localization
experts is that the total drag of these factors on programmers is such 
that they
tend to reach for `String.format` instead.

#### String Interpolation

Swift string interpolation provides a user-friendly alternative to 
printf's
domain-specific language (just write ordinary swift code!) and its type 
safety
problems (put the data right where it belongs!) but the following issues 
prevent
it from being useful for localized formatting (among other jobs):

  * [SR-2303](https://bugs.swift.org/browse/SR-2303) We are unable to 
restrict
    types used in string interpolation.
  * [SR-1260](https://bugs.swift.org/browse/SR-1260) String interpolation 
can't
    distinguish (fragments of) the base string from the string 
substitutions.

In the long run, we should improve Swift string interpolation to the point 
where
it can participate in most any formatting job.  Mostly this centers around
fixing the interpolation protocols per the previous item, and supporting
localization.

To be able to use formatting effectively inside interpolations, it needs 
to be
both lightweight (because it all happens in-situ) and discoverable.  One 
approach would be to standardize on `format` methods, e.g.:

```swift
"Column 1: \(n.format(radix:16, width:8)) *** \(message)"

"Something with leading zeroes: \(x.format(fill: zero, width:8))"
```

### C String Interop

Our support for interoperation with nul-terminated C strings is scattered 
and
incoherent, with 6 ways to transform a C string into a `String` and four 
ways to
do the inverse.  These APIs should be replaced with the following

```swift
extension String {
  /// Constructs a `String` having the same contents as 
`nulTerminatedUTF8`.
  ///
  /// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8 
encoded 
  ///   bytes ending just before the first zero byte (NUL character).
  init(cString nulTerminatedUTF8: UnsafePointer<CChar>)
 
  /// Constructs a `String` having the same contents as 
`nulTerminatedCodeUnits`.
  ///
  /// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code 
units in
  ///   the given `encoding`, ending just before the first zero code unit.
  /// - Parameter encoding: describes the encoding in which the code units
  ///   should be interpreted.
  init<Encoding: UnicodeEncoding>(
    cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
    encoding: Encoding)
 
  /// Invokes the given closure on the contents of the string, represented 
as a
  /// pointer to a null-terminated sequence of UTF-8 code units.
  func withCString<Result>(
    _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result
}
```

In both of the construction APIs, any invalid encoding sequence detected 
will
have its longest valid prefix replaced by U+FFFD, the Unicode replacement
character, per Unicode specification.  This covers the common case.  The
replacement is done *physically* in the underlying storage and the 
validity of
the result is recorded in the `String`'s `encoding` such that future 
accesses
need not be slowed down by possible error repair separately.

Construction that is aborted when encoding errors are detected can be
accomplished using APIs on the `encoding`.  String types that retain their
physical encoding even in the presence of errors and are repaired 
on-the-fly can
be built as different instances of the `Unicode` protocol.

### Unicode 9 Conformance

Unicode 9 (and MacOS 10.11) brought us support for family emoji, which 
changes
the process of properly identifying `Character` boundaries.  We need to 
update
`String` to account for this change.

### High-Performance String Processing

Many strings are short enough to store in 64 bits, many can be stored 
using only
8 bits per unicode scalar, others are best encoded in UTF-16, and some 
come to
us already in some other encoding, such as UTF-8, that would be costly to
translate.  Supporting these formats while maintaining usability for
general-purpose APIs demands that a single `String` type can be backed by 
many
different representations.

That said, the highest performance code always requires static knowledge 
of the
data structures on which it operates, and for this code, dynamic selection 
of
representation comes at too high a cost.  Heavy-duty text processing 
demands a
way to opt out of dynamism and directly use known encodings.  Having this
ability can also make it easy to cleanly specialize code that handles 
dynamic
cases for maximal efficiency on the most common representations.

To address this need, we can build models of the `Unicode` protocol that 
encode
representation information into the type, such as 
`NFCNormalizedUTF16String`.

### Parsing ASCII Structure

Although many machine-readable formats support the inclusion of arbitrary
Unicode text, it is also common that their fundamental structure lies 
entirely
within the ASCII subset (JSON, YAML, many XML formats).  These formats are 
often
processed most efficiently by recognizing ASCII structural elements as 
ASCII,
and capturing the arbitrary sections between them in more-general strings. 
 The
current String API offers no way to efficiently recognize ASCII and skip 
past
everything else without the overhead of full decoding into unicode 
scalars.

For these purposes, strings should supply an `extendedASCII` view that is 
a
collection of `UInt32`, where values less than `0x80` represent the
corresponding ASCII character, and other values represent data that is 
specific
to the underlying encoding of the string.

## Language Support

This proposal depends on two new features in the Swift language:

1. **Generic subscripts**, to
   enable unified slicing syntax.

2. **A subtype relationship** between
   `Substring` and `String`, enabling framework APIs to traffic solely in
   `String` while still making it possible to avoid copies by handling
   `Substring`s where necessary.

Additionally, **the ability to nest types and protocols inside
protocols** could significantly shrink the footprint of this proposal
on the top-level Swift namespace.


## Open Questions

### Must `String` be limited to storing UTF-16 subset encodings?

- The ability to handle `UTF-8`-encoded strings (models of `Unicode`) is 
not in
  question here; this is about what encodings must be storable, without
  transcoding, in the common currency type called “`String`”.
- ASCII, Latin-1, UCS-2, and UTF-16 are UTF-16 subsets.  UTF-8 is not.
- If we have a way to get at a `String`'s code units, we need a concrete 
type in
  which to express them in the API of `String`, which is a concrete type
- If String needs to be able to represent UTF-32, presumably the code 
units need
  to be `UInt32`.
- Not supporting UTF-32-encoded text seems like one reasonable design 
choice.
- Maybe we can allow UTF-8 storage in `String` and expose its code units 
as
  `UInt16`, just as we would for Latin-1.
- Supporting only UTF-16-subset encodings would imply that `String` 
indices can
  be serialized without recording the `String`'s underlying encoding.

### Do we need a type-erasable base protocol for UnicodeEncoding?

UnicodeEncoding has an associated type, but it may be important to be able 
to
traffic in completely dynamic encoding values, e.g. for “tell me the most
efficient encoding for this string.”

### Should there be a string “facade?”

One possible design alternative makes `Unicode` a vehicle for expressing
the storage and encoding of code units, but does not attempt to give it an 
API
appropriate for `String`.  Instead, string APIs would be provided by a 
generic
wrapper around an instance of `Unicode`:

```swift
struct StringFacade<U: Unicode> : BidirectionalCollection {

  // ...APIs for high-level string processing here...
 
  var unicode: U // access to lower-level unicode details
}

typealias String = StringFacade<StringStorage>
typealias Substring = StringFacade<StringStorage.SubSequence>
```

This design would allow us to de-emphasize lower-level `String` APIs such 
as
access to the specific encoding, by putting them behind a `.unicode` 
property.
A similar effect in a facade-less design would require a new top-level
`StringProtocol` playing the role of the facade with an an `associatedtype
Storage : Unicode`.

An interesting variation on this design is possible if defaulted generic
parameters are introduced to the language:

```swift
struct String<U: Unicode = StringStorage> 
  : BidirectionalCollection {

  // ...APIs for high-level string processing here...
 
  var unicode: U // access to lower-level unicode details
}

typealias Substring = String<StringStorage.SubSequence>
```

One advantage of such a design is that naïve users will always extend “the 
right
type” (`String`) without thinking, and the new APIs will show up on 
`Substring`,
`MyUTF8String`, etc.  That said, it also has downsides that should not be
overlooked, not least of which is the confusability of the meaning of the 
word
“string.”  Is it referring to the generic or the concrete type?

### `TextOutputStream` and `TextOutputStreamable`

`TextOutputStreamable` is intended to provide a vehicle for
efficiently transporting formatted representations to an output stream
without forcing the allocation of storage.  Its use of `String`, a
type with multiple representations, at the lowest-level unit of
communication, conflicts with this goal.  It might be sufficient to
change `TextOutputStream` and `TextOutputStreamable` to traffic in an
associated type conforming to `Unicode`, but that is not yet clear.
This area will require some design work.

### `description` and `debugDescription`

* Should these be creating localized or non-localized representations?
* Is returning a `String` efficient enough?
* Is `debugDescription` pulling the weight of the API surface area it 
adds?

### `StaticString`

`StaticString` was added as a byproduct of standard library developed and 
kept
around because it seemed useful, but it was never truly *designed* for 
client
programmers.  We need to decide what happens with it.  Presumably 
*something*
should fill its role, and that should conform to `Unicode`.

## Footnotes

<b id="f0">0</b> The integers rewrite currently underway is expected to
    substantially reduce the scope of `Int`'s API by using more
    generics. [↩](#a0)

<b id="f1">1</b> In practice, these semantics will usually be tied to the
version of the installed [ICU](http://icu-project.org) library, which
programmatically encodes the most complex rules of the Unicode Standard 
and its
de-facto extension, CLDR.[↩](#a1)

<b id="f2">2</b>
See
[http://unicode.org/reports/tr29/#Notation](
http://unicode.org/reports/tr29/#Notation). Note
that inserting Unicode scalar values to prevent merging of grapheme 
clusters would
also constitute a kind of misbehavior (one of the clusters at the boundary 
would
not be found in the result), so would be relatively costly to implement, 
with
little benefit. [↩](#a2)

<b id="f4">4</b> The use of non-UCA-compliant ordering is fully sanctioned 
by
  the Unicode standard for this purpose.  In fact there's
  a [whole chapter](http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf)
  dedicated to it.  In particular, §5.17 says:

  > When comparing text that is visible to end users, a correct linguistic 
sort
  > should be used, as described in _Section 5.16, Sorting and
  > Searching_. However, in many circumstances the only requirement is for 
a
  > fast, well-defined ordering. In such cases, a binary ordering can be 
used.

  [↩](#a4)


<b id="f5">5</b> The queries supported by `NSCharacterSet` map directly 
onto
properties in a table that's indexed by unicode scalar value.  This table 
is
part of the Unicode standard.  Some of these queries (e.g., “is this an
uppercase character?”) may have fairly obvious generalizations to grapheme
clusters, but exactly how to do it is a research topic and *ideally* we'd 
either
establish the existing practice that the Unicode committee would 
standardize, or
the Unicode committee would do the research and we'd implement their
result.[↩](#a5)

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