Re: Proposal: Partial Type Signatures - Status update

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

On Apr 10, 2014, at 3:48 AM, Thomas Winant 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:

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module Example where foo :: (Show _a, _) => _a -> _ foo x = show (succ x)

Compiled with GHC master gives:

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Example.hs:3:18: parse error on input ‘_’

When we compile it with our branch:

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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:

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[..] 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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