[swift-evolution] [Discussion] Analysis of the design of typed throws

Matthew Johnson matthew at anandabits.com
Thu Feb 23 12:04:28 CST 2017

> On Feb 23, 2017, at 11:53 AM, Vladimir.S <svabox at gmail.com> wrote:
> I'm really sorry to interrupt your discussion, but could someone describe(or point to some article etc) in two words why we need added complexity of typed throws(in comparing to use documentation)

Thrown errors already have an implicit type: `Error`.  What this proposal does is allow us to provide more specific types.

> and *if* the suggested solution will guarantee that some method can throw only explicitly defined type(s) of exception(s) including any re-thrown exception?

Yes, it handles this.  When more than one concrete error type is possible you will need to specify a common supertype or wrap them in an enum.  The suggested enhancement around implicit conversion during propagation will make this easier.  Until then we will need to manually wrap the errors.  I showed a pattern that can be used to do this with a reasonably small syntactic weight in functions that need to convert from one error type to another during propagation.

> The thread is really long and I personally was not able to follow it from the beginning(so I believe the answer can be helpful for others like me).
> Thank you(really).
> On 23.02.2017 20:09, Matthew Johnson via swift-evolution wrote:
>>> On Feb 23, 2017, at 10:58 AM, Anton Zhilin <antonyzhilin at gmail.com
>>> <mailto:antonyzhilin at gmail.com>> wrote:
>>> See some inline response below.
>>> Also, have you seen the issue I posted in Proposal thread? There is a way
>>> to create an instance of "any" type.
>> Yes, I saw that.  There is no problem with that at all.  As I point out in
>> the analysis below, rethrowing functions are allowed to throw any error
>> they want.  They are only limited by *where* they may throw.
>>> 2017-02-23 3:37 GMT+03:00 Matthew Johnson via swift-evolution
>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>>    # Analysis of the design of typed throws
>>>    ## Problem
>>>    There is a problem with how the proposal specifies `rethrows` for
>>>    functions that take more than one throwing function.  The proposal
>>>    says that the rethrown type must be a common supertype of the type
>>>    thrown by all of the functions it accepts.  This makes some intuitive
>>>    sense because this is a necessary bound if the rethrowing function
>>>    lets errors propegate automatically - the rethrown type must be a
>>>    supertype of all of the automatically propegated errors.
>>>    This is not how `rethrows` actually works though.  `rethrows`
>>>    currently allows throwing any error type you want, but only in a
>>>    catch block that covers a call to an argument that actually does
>>>    throw and *does not* cover a call to a throwing function that is not
>>>    an argument.  The generalization of this to typed throws is that you
>>>    can rethrow any type you want to, but only in a catch block that
>>>    meets this rule.
>>>    ## Example typed rethrow that should be valid and isn't with this
>>>    proposal
>>>    This is a good thing, because for many error types `E` and `F` the
>>>    only common supertype is `Error`.  In a non-generic function it would
>>>    be possible to create a marker protocol and conform both types and
>>>    specify that as a common supertype.  But in generic code this is not
>>>    possible.  The only common supertype we know about is `Error`.  The
>>>    ability to catch the generic errors and wrap them in a sum type is
>>>    crucial.
>>>    I'm going to try to use a somewhat realistic example of a generic
>>>    function that takes two throwing functions that needs to be valid
>>>    (and is valid under a direct generalization of the current rules
>>>    applied by `rethrows`).
>>>    enum TransformAndAccumulateError<E, F> {
>>>       case transformError(E)
>>>       case accumulateError(F)
>>>    }
>>>    func transformAndAccumulate<E, F, T, U, V>(
>>>       _ values: [T],
>>>       _ seed: V,
>>>       _ transform: T -> throws(E) U,
>>>       _ accumulate: throws (V, U) -> V
>>>    ) rethrows(TransformAndAccumulateError<E, F>) -> V {
>>>       var accumulator = seed
>>>       try {
>>>          for value in values {
>>>             accumulator = try accumulate(accumulator, transform(value))
>>>          }
>>>       } catch let e as E {
>>>          throw .transformError(e)
>>>       } catch let f as F {
>>>          throw .accumulateError(f)
>>>       }
>>>       return accumulator
>>>    }
>>>    It doesn't matter to the caller that your error type is not a
>>>    supertype of `E` and `F`.  All that matters is that the caller knows
>>>    that you don't throw an error if the arguments don't throw (not only
>>>    if the arguments *could* throw, but that one of the arguments
>>>    actually *did* throw).  This is what rethrows specifies.  The type
>>>    that is thrown is unimportant and allowed to be anything the
>>>    rethrowing function (`transformAndAccumulate` in this case) wishes.
>>> Yes, upcasting is only one way (besides others) to convert to a common
>>> error type. That's what I had in mind, but I'll state it more explicitly.
>> The important point is that if you include `rethrows` it should not place
>> any restrictions on the type that it throws when its arguments throw.  All
>> it does is prevent the function from throwing unless there is a dynamic
>> guarantee that one of the arguments did in fact throw (which of course
>> means if none of them can throw then the rethrowing function cannot throw
>> either).
>>>    ## Eliminating rethrows
>>>    We have discussed eliminating `rethrows` in favor of saying that
>>>    non-throwing functions have an implicit error type of `Never`.  As
>>>    you can see by the rules above, if the arguments provided have an
>>>    error type of `Never` the catch blocks are unreachable so we know
>>>    that the function does not throw.  Unfortunately a definition of
>>>    nonthrowing functions as functions with an error type of `Never`
>>>    turns out to be too narrow.
>>>    If you look at the previous example you will see that the only way to
>>>    propegate error type information in a generic function that rethrows
>>>    errors from two arguments with unconstrained error types is to catch
>>>    the errors and wrap them with an enum.  Now imagine both arguments
>>>    happen to be non-throwing (i.e. they throw `Never`).  When we wrap
>>>    the two possible thrown values `Never` we get a type of
>>>    `TransformAndAccumulateError<Never, Never>`.  This type is
>>>    uninhabitable, but is quite obviously not `Never`.
>>>    In this proposal we need to specify what qualifies as a non-throwing
>>>    function.  I think we should specifty this in the way that allows us
>>>    to eliminate `rethrows` from the language.  In order to eliminate
>>>    `rethrows` we need to say that any function throwing an error type
>>>    that is uninhabitable is non-throwing.  I suggest making this change
>>>    in the proposal.
>>>    If we specify that any function that throws an uninhabitable type is
>>>    a non-throwing function then we don't need rethrows.  Functions
>>>    declared without `throws` still get the implicit error type of
>>>    `Never` but other uninhabitable error types are also considered
>>>    non-throwing.  This provides the same guarantee as `rethrows` does
>>>    today: if a function simply propegates the errors of its arguments
>>>    (implicitly or by manual wrapping) and all arguments have `Never` as
>>>    their error type the function is able to preserve the uninhabitable
>>>    nature of the wrapped errors and is therefore known to not throw.
>>> Yes, any empty type should be allowed instead of just `Never`. That's a
>>> general solution to the ploblem with `rethrows` and multiple throwing
>>> parameters.
>> It looks like you clipped out the section "Why this solution is better”
>> which showed how `rethrows` is not capable of correctly typing a function
>> as non-throwing if it dynamically handles all of the errors thrown by its
>> arguments.  What do you think of that?  In my opinion, it makes a strong
>> case for eliminating rethrows and introducing the uninhabited type solution
>> from the beginning.
>>>    ### Language support
>>>    This appears to be a problem in search of a language solution.  We
>>>    need a way to transform one error type into another error type when
>>>    they do not have a common supertype without cluttering our code and
>>>    writing boilerplate propegation functions.  Ideally all we would need
>>>    to do is declare the appropriate converting initializers and
>>>    everything would fall into place.
>>>    One major motivating reason for making error conversion more
>>>    ergonomic is that we want to discourage users from simply propegating
>>>    an error type thrown by a dependency.  We want to encourage careful
>>>    consideration of the type that is exposed whether that be `Error` or
>>>    something more specific.  If conversion is cumbersome many people who
>>>    want to use typed errors will resort to just exposing the error type
>>>    of the dependency.
>>>    The problem of converting one type to another unrelated type (i.e.
>>>    without a supertype relationship) is a general one.  It would be nice
>>>    if the syntactic solution was general such that it could be taken
>>>    advantage of in other contexts should we ever have other uses for
>>>    implicit non-supertype conversions.
>>>    The most immediate solution that comes to mind is to have a special
>>>    initializer attribute `@implicit init(_ other: Other)`.  A type would
>>>    provide one implicit initializer for each implicit conversion it
>>>    supports.  We also allow enum cases to be declared `@implicit`.  This
>>>    makes the propegation in the previous example as simple as adding the
>>>    `@implicit ` attribute to the cases of our enum:
>>>    enum TransformAndAccumulateError<E, F> {
>>>       @implicit case transformError(E)
>>>       @implicit case accumulateError(F)
>>>    }
>>>    It is important to note that these implicit conversions *would not*
>>>    be in effect throughout the program.  They would only be used in very
>>>    specific semantic contexts, the first of which would be error
>>>    propegation.
>>>    An error propegation mechanism like this is additive to the original
>>>    proposal so it could be introduced later.  However, if we believe
>>>    that simply passing on the error type of a dependency is often an
>>>    anti-pattern and it should be discouraged, it is a good idea to
>>>    strongly consider introducing this feature along with the intial
>>>    proposal.
>>> Will add to Future work section.
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