#13594: Typechecker behavior w.r.t. BangPatterns and nested foralls has changed in 8.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Gah! GHC has two sorts of bindings: `FunBind` and `PatBind`: * `FunBind` is used for function bindings, obviously, but also for bindings of form `x = e`. As the comments in `HsBinds.hs` say: {{{ -- FunBind is used for both functions @f x = e@ -- and variables @f = \x -> e@ -- -- Reason 1: Special case for type inference: see 'TcBinds.tcMonoBinds'. -- -- Reason 2: Instance decls can only have FunBinds, which is convenient. -- If you change this, you'll need to change e.g. rnMethodBinds }}} * `PatBind` is used for all other pattern bindings So `!x = e` is treated as a `PatBind`. And that means that it will take the `InferGen` patch (see `TcBinds.decideGeneralisationPlan`); and that means that it will behave differently type-inference-wise than `x = e; x :: sig`. And that is bad: bangs aren't supposed to affect typing. Best solution (I think): add a bang-flag to `FunRhs` (c.f. the `LexicalFixity` flag) in `HsExpr.HsMatchContext`. Then * `!x = e` would be a "simple" pattern binding * `FunBind` would handle all simple pattern bindings (as well as true function binding) NB: `(x) = e` would not be a "simple" pattern binding, and would still go via `PatBind`. Maybe that's even a feature. We could instead put the flag in `FunBind` itself, but I'm influenced by that `LexicalFixity` flag, which Alan migraed from `FunBind` to the `HsMatchContext` place (I forget why). I'm too swamped work on this, but I'll happily offer guidance. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13594#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler