#11667: Incorrect pattern synonym types in error messages -------------------------------------+------------------------------------- Reporter: rdragon | Owner: rdragon Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc2 Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: #11213 Differential Rev(s): Phab:D1967 | Wiki Page: -------------------------------------+------------------------------------- Here are some examples of incorrect pattern synonym types in error messages. **Example 1** {{{#!hs {-# LANGUAGE PatternSynonyms #-} module Mod1 where pattern Pat :: Eq a => Maybe a pattern Pat <- Just 42 }}} This gives the error (note the missing `Eq a =>`): {{{ Mod1.hs:4:21: error: * Could not deduce (Num a) arising from the literal `42' from the context: Eq a bound by the type signature for pattern synonym `Pat': Maybe a at Mod1.hs:4:9-11 Possible fix: add (Num a) to the context of the type signature for pattern synonym `Pat': Maybe a * In the pattern: 42 In the pattern: Just 42 In the declaration for pattern synonym `Pat' }}} **Example 2** {{{#!hs {-# LANGUAGE PatternSynonyms, GADTs #-} module Mod1 where data T where MkT :: a -> T pattern Pat :: () => b ~ Bool => a -> b -> (b, T) pattern Pat x y = (y, MkT x) }}} We get the error (note the missing contexts and the incorrect quantification `forall b`, missing `a`): {{{ Mod1.hs:6:27: error: * Couldn't match type `b' with `Bool' arising from the "provided" constraints claimed by the signature of `Pat' `b' is a rigid type variable bound by the type signature for pattern synonym `Pat': forall b. a -> b -> (b, T) at Mod1.hs:5:16 * In the declaration for pattern synonym `Pat' * Relevant bindings include y :: b (bound at Mod1.hs:6:20) }}} **Example 3** {{{#!hs {-# LANGUAGE PatternSynonyms #-} module Mod1 where pattern Pat :: Eq a => Show a => a -> Maybe a pattern Pat x <- Just x }}} We get the error: {{{ Mod1.hs:4:23: error: * Could not deduce (Show a) arising from the "provided" constraints claimed by the signature of `Pat' from the context: Eq a bound by the type signature for pattern synonym `Pat': a -> Maybe a at Mod1.hs:4:9-11 * In the declaration for pattern synonym `Pat' }}} Here we could perhaps remove the whole "from the context ... at Mod1.hs:4:9-11" part, as it doesn't make much sense to tell the user that we could not deduce a "provided" constraint from the "required" context. **Example 4** {{{#!hs {-# LANGUAGE PatternSynonyms, GADTs #-} module Mod1 where data T a where MkT :: (Num a, Show a) => a -> T a pattern Pat :: (Eq a) => (Show a) => T a pattern Pat = MkT 42 }}} This gives the error (note that the signature in the error is of the pattern synonym `Pat` when used in an expression context): {{{ Mod1.hs:6:15: error: * Could not deduce (Num a) arising from a use of `MkT' from the context: (Eq a, Show a) bound by the type signature for pattern synonym `Pat': (Eq a, Show a) => T a at Mod1.hs:6:1-20 Possible fix: add (Num a) to the context of the type signature for pattern synonym `Pat': (Eq a, Show a) => T a * In the expression: MkT 42 In an equation for `$bPat': $bPat = MkT 42 }}} The problems in these four examples are caused by the use of `SigSkol (PatSynCtxt n) (Check ty)` for representing pattern synonym types (and the types of bidirectional pattern synonyms when used in an expression context). A possible solution can be found in Phab:D1967. Simon notes the following:

Example 3 raises an interesting point we can debate once we have a ticket. Namely, is this ok? {{{#!hs pattern Pat :: Ix a => Ix a => a -> Maybe a pattern Pat x <- Just x }}}

-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11667 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler