#14570: Untouchable error arises from type equality, but not equivalent program with fundeps -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Resolution: | Keywords: 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 AntC): Replying to [comment:3 dfeuer]:
At the least, this is a rather poor error message.
I'm wondering if the o.p. gave the full story about the error message. I retried the Type Family version, and got `f0 is untouchable` rejection, but against the type for `f`. The message went on to give more info `f is a rigid type variable ...`; plus a suggestion to `AllowAmbiguousTypes`. Switching that on does indeed suppress the message against `f`; then it just reappears against `g`, as per the o.p., but with less helpful info. (I've put more narrative against the StackOverflow question, link above.) Simon's explanation at comment:1 tells what's going on, but not really why. The Schrijvers et al paper didn't really help: it's aiming to explain FunDep inference in terms of Type Families/System FC, so doesn't tell why/how ghc's behaviour is different in this example. Re the `rigid type variable` message, there are some reasonable explanations on StackOverflow. Re `f0 is untouchable` not so much. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14570#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler