Proposal: Partial Type Signatures - Status update

[Richard Eisenberg]

Yay!

I have nothing else constructive to say, at the moment.

What's the next step from your point of view? Are there unimplemented bits of this?

Thanks for doing this!
Richard

On Apr 10, 2014, at 3:48 AM, Thomas Winant <Thomas.Winant at cs.kuleuven.be> wrote:

> Hi,
> 
> I'm back with a status update. We implemented Austin's suggestion to
> make wildcards in partial type signatures behave like holes.
> 
> Let's demonstrate the new behaviour with an example. The example
> program:
> 
>> module Example where
>> foo :: (Show _a, _) => _a -> _
>> foo x = show (succ x)
> 
> Compiled with GHC master gives:
> 
>> Example.hs:3:18: parse error on input ‘_’
> 
> When we compile it with our branch:
> 
>> Example.hs:3:18:
>>    Instantiated extra-constraints wildcard ‘_’ to:
>>      (Enum _a)
>>      in the type signature for foo :: (Show _a, _) => _a -> _
>>      at Example.hs:3:8-30
>>    The complete inferred type is:
>>      foo :: forall _a. (Show _a, Enum _a) => _a -> String
>>    To use the inferred type, enable PartialTypeSignatures
>> Example.hs:3:30:
>>    Instantiated wildcard ‘_’ to: String
>>      in the type signature for foo :: (Show _a, _) => _a -> _
>>      at Example.hs:3:8-30
>>    The complete inferred type is:
>>      foo :: forall _a. (Show _a, Enum _a) => _a -> String
>>    To use the inferred type, enable PartialTypeSignatures
> 
> Now the types the wildcards were instantiated to are reported. Note that
> `_a` is still treated as a type variable, as prescribed in Haskell 2010.
> To treat it as a /named wildcard/, we pass the -XNamedWildcards flag and
> get:
> 
>> [..]
>> Example.hs:3:24:
>>    Instantiated wildcard ‘_a’ to: tw_a
>>      in the type signature for foo :: (Show _a, _) => _a -> _
>>      at Example.hs:3:8-30
>>    The complete inferred type is:
>>      foo :: forall tw_a. (Show tw_a, Enum tw_a) => tw_a -> String
>>    To use the inferred type, enable PartialTypeSignatures
>> [..]
> 
> An extra error message appears, reporting that `_a` was instantiated to
> a new type variable (`tw_a`).
> 
> Finally, when passed the -XPartialTypeSignatures flag, the typechecker
> will just use the inferred types for the wildcards and compile the
> program without generating any error messages.
> 
> We added this example and a section about the monomorphism restriction
> to the wiki page [1].
> 
> Comments and feedback are still welcome.
> 
> Cheers,
> Thomas Winant
> 
> 
> [1]: https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures
> 
> 
> 
> 
> 
> 
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