#12187: Clarify the scoping of existentials for pattern synonym signatures -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #12108 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I propose the following: * The universal type variables are the ones free in * any explicitly-forall'd universal variables * the required context (if any) * the result type, after deleting any explicitly-forall'd existentials. * For this purpose the "result type" is very narrowly interpreted, as follows: first drop the required and provided contexts, if present. Now drop arrows (only). What's left is the result type. In this suggestion, even parentheses stop the "dropping arrows" bit. For example: {{{ pattern X :: a -> (b -> T a) }}} Here the "result type" is `(b -> T a)`. * The existentials are all the rest This rule is simple and predictable. For `Q` in the description, both `a` and `b` are universal, because the result type is `Disguised a b` and both `a` and `b` are free. Moreover, you can always override it by using explicit foralls, to get what you want. E.g. {{{ pattern Q :: () => forall b. Eq b => Disguised a b }}} The `forall` means that `b` is no longer free in the result type Yes we could "look through" parens. Yes, we could "look through" type synonyms, although that is harder because scopging is determined in the renamer. But it's a slippery slope; e.g. what about type functions? I say keep it simple. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12187#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler