[Haskell-cafe] Confused about AllowAmbiguousTypes

[Roman Cheplyaka]

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I imagine defaulting is involved here.

On 09/18/2014 06:00 PM, Audun Skaugen wrote:
> I am confused about the behaviour of the AllowAmbiguousTypes
> extension. I tried the basic example of the user manual:
> 
>> class C a where f :: C a => Int f = 3 main = print f
> 
> Using ghc 7.8.3, without the extension, this will cause a type
> error at the type signature (line 2). Adding AllowAmbgiuousTypes
> causes an error at the call site instead (line 4). So far so good.
> 
> However, if I replace the (C a) constraint with (Num a), no type
> error is reported, and the example runs fine! This is true with or
> without the extension. Using ghc 7.6.3, however, the example is
> still rejected with an "ambiguous constraint" error.
> 
> Is this a bug in ghc 7.8? Is ghc somehow able to prove the Num 
> constraint even though a is universally quantified?
> 
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