#10794: Extension request: "where" clauses in type signatures -------------------------------------+------------------------------------- Reporter: danso | Owner: Type: feature request | Status: new Priority: lowest | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:4 simonpj]:
{{{ fold :: (f ~ a -> a -> a) => f -> [a] -> [a] }}}
But doing this changes type inference characteristics, by introducing an equality and making some type variables untouchable. This was pointed out in an earlier bug (sadly, I can't remember any search terms) that highlighted a case where the static semantics of using this sort of trick was different than the expanded form. The place I've wanted a feature like this most is when writing intricate type families. But there, it would be necessary to allow local type family definitions in the `where` clause. All that said, I'm ambivalent about this proposal as stated. It would be useful. But does Simon's trick work often enough? Is it worth specifying, developing, and maintaining this? I'm not sure. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10794#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler