The exhaustiveness checker currently chokes on pattern synonyms. They are marked as always fallible patterns which means that we must also always include a catch-all case in order to avoid a warning.
{{{#!hs data A = A
pattern :: A pattern P = A
foo :: A -> A foo P = A }}}
leads to the warning that pattern matches for `foo` are non-exhaustive.
{{{ simpletest.hs:7:1: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In an equation for ‘foo’: Patterns not matched: _ }}}
Inspecting the definition of `P` we can see that the matches for `foo` are indeed exhaustive as `A` is a unary data type but the pattern match checker does not make use of this information.
And neither should it! Pattern synonyms are a means of *abstraction*, if
We want users to be able to replace bona-fide data constructors with
#15886: Spurious warning about incomplete pattern with PatternSynonyms -------------------------------------+------------------------------------- Reporter: selinger | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.1 Resolution: invalid | Keywords: | PatternSynonyms, | PatternMatchWarnings Operating System: Linux | Architecture: x86_64 Type of failure: Incorrect | (amd64) error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): The rationale for requiring `COMPLETE` signatures to coverage-check pattern synonyms can be found in [https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms/CompleteSigs?version=1... the corresponding GHC wiki page]: the exhaustiveness checker could look through a definition then the implementation of `P` would leak into error messages. pattern synonyms without consumers noticing.
To that end, we allow users to specify a complete set of pattern synonyms in order to sate the pattern match checker. If a complete pragma is not provided then we keep the same behaviour as in previous releases.
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15886#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler