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 Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm