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.win...@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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