#12949: Pattern coverage mistake -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: gkaracha Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
Indeed you are right, this is not a bug in the type constraints that the checker generates but the term constraints. I expected the knownBools to be already desugared to (knownBool $dKnownBool_a) and (knownBool $dKnownBool_b) since they are overloaded but I was mistaken.
I'm going to need more guidance before I can help. Can you give instructions to reproduce the bit you think is unexpected? I compiled the program (adding a definition for `Dict` which you omitted), with `-ddump-ds` and got {{{ ... ds_d3Xn = knownBool @ a $dKnownBool_a3Ux } in ... ds_d3Xo = knownBool @ b $dKnownBool_a3UL } in }}} which is what you expect. This is post-desugaring. Before desugaring there'll be a `HsWrapper` involved. Anyway, please show me specifically what you think it unexpected. ---------------- Incidentally, the constraint signatures don't seem to work. Compiling {{{ type T a = Eq a :: Constraint }}} yields a syntax error, as does {{{ type T a :: Constraint = Eq a }}} I must be doing something stupid. But what? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12949#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler