#14694: Can't coerce given assumptions -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Roles Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by goldfire): * keywords: => Roles Comment: This is yet another incompleteness in the solver. A given like `cat a b ~R (a -> f b)` cannot be decomposed, as the typing rules for roles forbids this. Currently, GHC remembers the given and uses it only if a wanted matches the given exactly. Thus, this ticket. One approach would be to have some structure that remembers `cat a b` maps to `a -> f b` and then use this to look up types. Actually, this wouldn't even be all that hard: the left-hand sides would all be `AppTy`s (as plain old tyvars already have their own mechanism: `CTyEqCan`), and I believe just handling a top-level `AppTy` would be enough to make this approach complete. (I don't believe we'd ever need to look for a nested `AppTy` -- say, as the argument to a tycon -- because we'll decompose larger types until the `AppTy` bubbles up to the top.) We could just build a `TrieMap` mapping types to types; put a new entry in the `TrieMap` on given `AppTy` equalities and look up in the `TrieMap` on wanteds. Perhaps there's more to it than this, but I don't really think so. Iceland_jack, do you remember if there's a place where we collect representational incompletenesses? This is not the first. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14694#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler