Hi all, I'm writing a short function as follows, but I'm not able to find a suitable type signature for `go'. It uses Numeric.LinearAlgebra from hmatrix. -- | Map each element in a vector to vectors and thus form a matrix -- | row by row mapVecToMat :: (Element a, Element b) => (a -> Vector b) -> Vector a -> Matrix b mapVecToMat f v = fromRows $ go (d - 1) [] where d = dim v go :: Element b => Int -> [Vector b] -> [Vector b] go 0 vs = f (v @> 0) : vs go !j !vs = go (j - 1) (f (v @> j) : vs) If I give the type signature to go as this, I got the following error Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the type signature for `go' at test.hs:36:20 `b' is a rigid type variable bound by the type signature for `mapVecToMat' at test.hs:31:35 Expected type: Vector b1 Inferred type: Vector b In the first argument of `(:)', namely `f (v @> 0)' In the expression: f (v @> 0) : vs So what is this rigid type variable all about and what is correct type of the function `go'? Thanks in advance, X-Y -- c/* __o/* <\ * (__ */\ <