Hi, In Chapter 6 of "Beginning Haskell" by Apress there's a couple of classes introduced for vectors and things that can be vectorized (not related to Data.Vector) {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} class Vector v where distance :: v -> v -> Double instance Vector (Double, Double) where distance (a,b) (c,d) = sqrt $ (c - a) * (c - a) + (d - b) * (d - b) class Vectorizable e v where toVector :: e -> v instance Vectorizable (Double, Double) (Double, Double) where toVector = id x = 1.0 :: Double y = 10.0 :: Double While I understand how to use the "distance" function: ghci> distance (x, x) (y, y) 12.727922061357855 ... and I can see how "toVector" is used in their code ghci> distance (x, x) $ toVector (y, y) 12.727922061357855 I don't understand why this doesn't work: ghci> let z = toVector (y, y) interactive>:32:9: No instance for (Vectorizable (Double, Double) v0) arising from a use of `toVector' The type variable `v0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there is a potential instance available: instance Vectorizable (Double, Double) (Double, Double) -- Defined at vector.hs:13:10 Possible fix: add an instance declaration for (Vectorizable (Double, Double) v0) In the expression: toVector (y, y) In an equation for `z': z = toVector (y, y) It seems odd that "toVector" works when used as an argument to "distance" but not when used in a let expression. Can anyone explain? Derek.