#14552: GHC panic on pattern synonym -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 Resolution: | Keywords: | PatternSynonyms, TypeInType, | ViewPatterns Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Let's make it even simpler. Use Richard's data type `T`, and consider {{{ {-# LANGUAGE ViewPatterns, RankNTypes, GADTs #-} module Foo where data T where T :: (forall a. Int -> a -> a) -> T g1 (T f) = (f 3 'c', f 3 True) g2 (T f) = let h = f 3 in (h 'c', h True) g3 (T (id -> f)) = (f 3 'c', f 3 True) g4 (T ((\f -> f 3) -> h)) = (h 'c', h True) }}} Here we get * `g1` typechecks because `f` is bound to a polymorphic function. * `g2` does not typecheck, because the binding of `h` is not generalised (we have `MonoLocalBinds` when `GADTs` is on). So `h` is monomorphic. * `g3` does not typecheck for the same reason. It's akin to `g2` with `h = id f`; it desugars to something like {{{ g3 (T ff) = let f = id ff in (f 3 'c', f 3 True) }}} * `g4` does not typecheck for the same reason. It desugars to something like {{{ g4 (T ff) = let h = (\f -> f 3) ff in (h 'c', h True) }}} So I don't think this has anything to do with the impredicativity magic for `($)`. It's just that binding a polymorphic variable in a pattern is a very delicate business. One could imagine generalising those extra let-bindings in the desugarings of `g3` and `g4`, but would be hard to do reliably -- that is why we have `MonoLocalBinds`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14552#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler