#15991: Regression in error message when attempting to let bind an existentially quantified type -------------------------------------+------------------------------------- Reporter: mmailhot | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.4.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When attempting to compile the following (invalid) program: {{{#!hs {-# LANGUAGE ExistentialQuantification #-} data Foo = forall a. Foo a main :: IO () main = let Foo x = Foo 1 in return () }}} GHC 8.6.2.0 (and 8.6.1.0, 8.4.1.0) gives the following complicated error message {{{ Test.hs:7:13: error: • Couldn't match expected type ‘p’ with actual type ‘a’ because type variable ‘a’ would escape its scope This (rigid, skolem) type variable is bound by a pattern with constructor: Foo :: forall a. a -> Foo, in a pattern binding at Test.hs:7:9-13 • In the pattern: Foo x In a pattern binding: Foo x = Foo 1 In the expression: let Foo x = Foo 1 in return () | 7 | let Foo x = Foo 1 in | }}} GHC 7.10.1.2 gave a much more helpful and direct error message {{{ Test.hs:7:9: My brain just exploded I can't handle pattern bindings for existential or GADT data constructors. Instead, use a case-expression, or do-notation, to unpack the constructor. In the pattern: Foo x In a pattern binding: Foo x = Foo 1 In the expression: let Foo x = Foo 1 in return () }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15991 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler