Allow ambiguous types (with warning) by default

[Edward Kmett]

If you aren't the one writing the code that can't be called you may never
see the warning. It'll be tucked away in a cabal or stack build log
somewhere.

-Edward

On Sun, Dec 6, 2015 at 12:06 AM, David Feuer <david.feuer at gmail.com> wrote:

> No, I want it to *warn* by default. If I write
>
> foo :: something that will fail the ambiguity check
> bar = something that uses foo in a (necessarily) ambiguous way
>
> the current default leads me to do this:
>
> 1. Attempt to compile. Get an ambiguity error on foo whose exact cause
> is hard for me to see.
> 2. Enable AllowAmbiguousTypes and recompile. Get an error on bar whose
> exact cause is completely obvious, and that makes it perfectly clear
> what I need to do to fix foo.
> 3. Fix foo, and disable AllowAmbiguousTypes.
>
> I'd much rather go with
>
> 1. Attempt to compile. Get an ambiguity *warning* on foo whose exact
> cause is hard for me to see, but also an error on bar whose exact
> cause is completely obvious, and that makes it perfectly clear what I
> need to do to fix foo.
> 2. Fix foo.
>
> Simple example of how it is currently:
>
> > let foo :: Num a => F a; foo = undefined; bar :: Int; bar = foo
>
> <interactive>:14:12:
>     Couldn't match expected type ‘F a’ with actual type ‘F a0’
>     NB: ‘F’ is a type function, and may not be injective
>     The type variable ‘a0’ is ambiguous
>     In the ambiguity check for the type signature for ‘foo’:
>       foo :: forall a. Num a => F a
>     To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
>     In the type signature for ‘foo’: foo :: Num a => F a
>
> Couldn't match what with what? Huh? Where did a0 come from?
>
> > :set -XAllowAmbiguousTypes
> > let foo :: Num a => F a; foo = undefined; bar :: Int; bar = foo
>
> <interactive>:16:61:
>     Couldn't match expected type ‘Int’ with actual type ‘F a0’
>     The type variable ‘a0’ is ambiguous
>     In the expression: foo
>     In an equation for ‘bar’: bar = foo
>
> Aha! That's the problem! It doesn't know what a0 is! How can I tell it
> what a0 is? Oh! I can't, because foo doesn't give me a handle on it.
> Guess I have to fix foo.
>
> I'd really, really like to get *both* of those messages in one go,
> with the first one preferably explaining itself a bit better.
>
> On Sat, Dec 5, 2015 at 11:51 PM, Edward Kmett <ekmett at gmail.com> wrote:
> > So you are saying you want users to write a ton of code that happens to
> have
> > signatures that can never be called and only catch it when they go to
> try to
> > actually use it in a concrete situation much later?
> >
> > I don't really show how this would be a better default.
> >
> > When and if users see the problem later they have to worry about if they
> are
> > doing something wrong at the definition site or the call site. With the
> > status quo it complains at the right time that you aren't going to sit
> there
> > flailing around trying to fix a call site that can never be fixed.
> >
> > -Edward
> >
> > On Sat, Dec 5, 2015 at 5:38 PM, David Feuer <david.feuer at gmail.com>
> wrote:
> >>
> >> The ambiguity check produces errors that are quite surprising to the
> >> uninitiated. When the check is suppressed, the errors at use sites are
> >> typically much easier to grasp. On the other hand, there's obviously a
> lot
> >> of value to catching mistakes as soon as possible. Would it be possible
> to
> >> turn that into a warning by default?
> >>
> >>
> >> _______________________________________________
> >> ghc-devs mailing list
> >> ghc-devs at haskell.org
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >>
> >
>
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