#13537: Adding underscore (m :: _) to pattern variable makes it fail -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by Iceland_jack): * status: new => closed * resolution: => invalid Comment: Replying to [comment:2 simonpj]:
Now you want to force the type of the pattern to be `_`. But since GHC only supports predicative polymorphism, `_` must be a monotype. So GHC instantiates `forall a. a->a` to `alpha -> alpha` (for some new unification variable alpha), and proceeds.
I suspected this was related to impredicativity. As a user I expected transforming the pattern `m` to `(m :: _)` would have no effect since it binds nothing (the compiler ''could'' special-case it, even though it brings no benefits).
Does that help?
Is there some way we could improve the user manual?
Yes it helps. I think the impredicative polymorphism entry is rather good, thanks for the explanation. To be clear, this is also expected? Closing the ticket {{{#!hs -- Works tId :: T -> T tId (MkT g) = MkT g -- Fails tId :: T -> T tId (MkT g) = MkT g' where g' = g }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13537#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler