Proposal: Partial Type Signatures - Status update

[Thomas Winant]

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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