#7857: GHC says there are unsafe overlapping instances, but only one instance applies -----------------------------+---------------------------------------------- Reporter: dmwit | Owner: Type: bug | Status: new Priority: normal | Component: Compiler Version: 7.6.1 | Keywords: Os: Unknown/Multiple | Architecture: Unknown/Multiple Failure: None/Unknown | Blockedby: Blocking: | Related: -----------------------------+---------------------------------------------- The following code rightfully does not compile: {{{ import Text.Printf f :: a -> b f = undefined g i = f $ printf "" i }}} However, the error message is a very surprising one: {{{ test.hs:6:1: Unsafe overlapping instances for PrintfType (t -> a0) arising from the ambiguity check for `g' The matching instance is: instance [safe] (PrintfArg a, PrintfType r) => PrintfType (a -> r) -- Defined in `Text.Printf' It is compiled in a Safe module and as such can only overlap instances from the same module, however it overlaps the following instances from different modules: When checking that `g' has the inferred type `forall a b t. PrintfType (t -> a) => t -> b' Probable cause: the inferred type is ambiguous }}} What I find surprising is the claim that there are overlapping instances, followed by a list of instances that clearly does not have any overlaps. For comparison, changing the definition of g to be {{{ g = f . printf "" }}} gives a much more reasonable error: {{{ test.hs:6:9: No instance for (PrintfArg a0) arising from a use of `printf' The type variable `a0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance [safe] PrintfArg Char -- Defined in `Text.Printf' instance [safe] PrintfArg Double -- Defined in `Text.Printf' instance [safe] PrintfArg Float -- Defined in `Text.Printf' ...plus 12 others In the second argument of `(.)', namely `printf ""' In the expression: f . printf "" In an equation for `g': g = f . printf "" }}} -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7857 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler