#13649: RebindableSyntax causes type errors when 'fail' is not defined, even if not used. -------------------------------------+------------------------------------- Reporter: AaronFriel | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc1 Resolution: | Keywords: | RebindableSyntax Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): This is not really a bug, but rather how `do` notation is specified. If you look at https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-470003.1... you will see {{{ do {p <- e; stmts} = let ok p = do {stmts} ok _ = fail "..." in e >>= ok }}} One could argue that if the pattern is `_`, the second line should be omitted, but where to stop? What about `do () <- …`, an obviously complete pattern, should the `fail` line be omitted as well? But that information might only be available after type-checking, whereas this happens before… I don’t have an opinion of my own here, just pointing out that the current behaviour is as specified, and that a change is not as trivial as it looks. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13649#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler