[swift-evolution] Proposal: Rewrite Swift compiler in swift to get ideas for further language evolution.

[Andrew Bennett]

My code is often abstract and full of generics, but the standard library
code does do several things that aren't the swift we all know and love,
from my brief tinkering I've seen at least these things:
 * It defines public protocols dependent on private protocols like
_MaxBuiltinIntegerType
 * It uses special type annotations like @_transparent
 * It extensively uses macro-like files (See FixedPoint.swift.gyb)
 * The Builtin module, where do I find that?

I think to a certain extent these things are probably necessary to get the
work done and make it flexible, and I'm sure the need for them will
decrease over time. However it does mean that the dev team doesn't have the
same restrictions on design and implementation that we do. This also means
that the dev team is essentially using a different version of Swift and
that will inform their decisions on what Swift needs and how it should be
used.

As for writing the compiler in Swift, I think this would be great, I
haven't read Colin's article yet but it sounds like a valid concern.

It's a huge undertaking and perhaps something that can be done
incrementally by the community as well. Chris Lattner has mentioned in the
past that many of the devs working on Swift would love to do this:

>From his twitter (
https://twitter.com/clattner_llvm/status/613906970890801152):

@*siracusa* Many of us would love to rewrite the swift compiler in swift -
it would crash a lot less, and be a lot more joyful for us.

@*siracusa* that said, we have a ton of other higher priorities that affect
users of swift.  Poor compiler hackers just have to suffer for now

<https://twitter.com/clattner_llvm/status/613906586050826241>


On Sun, Dec 20, 2015 at 11:40 AM, Colin Barrett via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Dec 19, 2015, at 7:39 PM, Amir Michail <a.michail at me.com> wrote:
>
>
> On Dec 19, 2015, at 7:37 PM, Colin Barrett <colin at springsandstruts.com>
> wrote:
>
>
> On Dec 19, 2015, at 7:32 PM, Amir Michail <a.michail at me.com> wrote:
>
>
> On Dec 19, 2015, at 7:21 PM, Colin Barrett <colin at springsandstruts.com>
> wrote:
>
> I’d recommend you read
> http://tratt.net/laurie/blog/entries/the_bootstrapped_compiler_and_the_damage_done,
> which has a number of rebuttals to what you’ve said below.
>
>
> That’s an interesting article but it doesn’t address the issue of whether
> compiler code is more like normal programming than compiler standard
> library code.
>
>
> Perhaps I don’t understand what you mean, but the article gives two good
> reasons why compiler code is special.
>
>
> Compiler standard library code tends to be very abstract and full of
> generics. Normal code isn’t like that.
>
>
> Speak for yourself ;-)
>
>
> The first reason is that we understand a lot about how to design a
> compiler, much more than we understand about how to design other types of
> programs. The second follows:
>
> [C]ompilers are an atypical class of program. In essence, a compiler is a
> simple batch pipeline process. A program is read in and translated to a
> tree; a series of tree transformations are applied; and eventually one of
> those trees is saved out as some sort of binary data (e.g. machine code or
> bytecode). Most of the intermediate tree transformations calculate a
> relatively simple bit of information about the program and create a
> slightly modified tree based on it. A few calculations crop up time and
> time again, such as: maps from variables to scopes or types; and stacks to
> determine closures. Significantly, and unlike most programs in the real
> world, there is no interaction with users: the compiler knows all it needs
> about the outside world from the moment it is called.
>
>
> Personally, I think the main reason not to rewrite the Swift compiler is
> that it would be a distraction from improving the Swift language and other
> associated tools.
>
> -Colin
>
> On Dec 19, 2015, at 4:41 PM, Amir Michail via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Compiler code is probably more typical of what most programmers write than
> library code and so would be ideal for suggesting further language
> evolution ideas.
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