Dear everyone, I have a code https://github.com/nushio3/practice/blob/master/instance-inference/zipf-11-1... that produces a type-error when I remove a type signature. https://github.com/nushio3/practice/blob/master/instance-inference/zipf-11.h... The strange point is the following error message from ghc 7.6.1 (abbreviated; the full error text is included as the comment of the code): No instance for (PType (Cons V.Vector (V.Vector Double) (Cons V.Vector (V.Vector Char) (Cons V.Vector (V.Vector Int) (Nil V.Vector)))) ((Double -> Char -> Int -> String) -> a0)) arising from a use of `forZN' The type variable `a0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there is a potential instance available: instance [overlap ok] (Zip v, Reduce v f0 vaS r (Nil v)) => PType (Cons v (v i) vaS) ((i -> f0) -> v r) ghc presents only one potential instance, and it seems to me that by setting v = V.Vector i = Double f0 = Char -> Int -> String r = String etc, you can actually match the 'potential' instance. The fact that this code typechecks with an additional type signature (zipf-11-1.hs) shows that there is a way to satisfy the constraint. So the question: why the instance inference stops here, when there is only one potential instance and no ambiguity? Or is it? What are potential instances? I thought the cause of the error is that ghc cannot match (v r) with (a0), and so I tried to shrink the example before I post. However my guess was wrong. Tinyer example did typecheck: https://github.com/nushio3/practice/blob/master/instance-inference/copy-01.h... https://github.com/nushio3/practice/blob/master/instance-inference/copy-02.h... https://github.com/nushio3/practice/blob/master/instance-inference/copy-04.h... If you have any sample codes that fails to typecheck and presents only one "potential instance" , that will be a great clue for me. Also, please recommend me any reading material for instance inference. (Haskell 98 deals with instance declaration syntax, and there are variety of text for type inference, but I could find few for instance inference.) Best, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html