Proposal: Partial Type Signatures - Status update

[Thomas Winant]

Hi,

My apologies for the late reply.

On 2014-04-10 17:43, Richard Eisenberg wrote:
> What's the next step from your point of view? Are there unimplemented
> bits of this?

We do see some bits left to implement:
* Partial pattern and expression signatures (see [1] for our view on
   this issue).
* Partial type signatures for local bindings. What with 'let should not
   be generalised' (see [2])?

The implementation also needs a thorough code review (and probably some
refactoring as well) from a GHC dev.

We'll be talking to SPJ via Skype on Thursday to discuss further
details. I'll post another status update in due course.


Cheers,
Thomas Winant


[1]: 
https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures#PartialExpressionandPatternSignatures
[2]: 
https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures#LocalDefinitions



> 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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