#14395: Redefining pattern synonym in GHCi triggers "‘p’ is untouchable" error -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Load this file into GHCi: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} module Bug where data Foo a where FooInt :: Foo Int pattern Bar :: () => (a ~ Int) => Foo a pattern Bar = FooInt }}} And attempt to redefine `Bar` as follows: {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, 1 module loaded. λ> pattern Bar = Nothing <interactive>:1:15: error: • Couldn't match expected type ‘p’ with actual type ‘Maybe a0’ ‘p’ is untouchable inside the constraints: a ~ Int bound by a pattern with pattern synonym: Bar :: forall a. () => a ~ Int => Foo a, in an equation for ‘pattern’ at <interactive>:1:9-11 ‘p’ is a rigid type variable bound by the inferred type of pattern :: Foo a -> p at <interactive>:1:1-21 Possible fix: add a type signature for ‘pattern’ • In the expression: Nothing In an equation for ‘pattern’: pattern Bar = Nothing • Relevant bindings include pattern :: Foo a -> p (bound at <interactive>:1:1) }}} There are two issues here: 1. There are several places in the error message that refer to a `pattern` with no name! {{{ in an equation for ‘pattern’ }}} {{{ the inferred type of pattern :: Foo a -> p at <interactive>:1:1-21 }}} {{{ • Relevant bindings include pattern :: Foo a -> p (bound at <interactive>:1:1) }}} 2. Why is this error happening in the first place? The error message mentions the type `Foo a -> p`, but in `pattern Bar = Nothing`, there isn't anything that should touch `Foo`. Note that this bug does not occur if a slightly different (but ostensibly equivalent) type signature for `Bar` is given in the original source file: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} module Works where data Foo a where FooInt :: Foo Int pattern Bar :: Foo Int pattern Bar = FooInt }}} {{{ λ> pattern Bar = Nothing λ> :i Bar pattern Bar :: Foo Int }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14395 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler