Allow ambiguous types (with warning) by default

[David Feuer]

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?
>>
>>
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>