The following code is intended as a first step towards creating a cyclical enumerable type, such that: (e.g.) [Cyc Friday .. Cyc Tuesday] would yield [Friday, Saturday, Sunday, Monday, Tuesday] module Cycle where newtype Cyc a = Cyc a deriving (Eq, Ord, Bounded, Show, Read) fromCyc :: Cyc a -> a fromCyc (Cyc a) = a instance (Enum a, Bounded a) => Enum (Cyc a) where fromEnum = fromEnum . fromCyc toEnum n = Cyc x where (x, max) = (x', maxBound) :: (a, a) x' = toEnum $ n `mod` ((fromEnum max) - 1) This yields a kind of error message that I've often bashed my head against in other code I've written, without ever really understanding what the problem is exactly: Couldn't match type `a0' with `a1' because type variable `a1' would escape its scope This (rigid, skolem) type variable is bound by an expression type signature: (a1, a1) The following variables have types that mention a0 x' :: a0 (bound at Cycle.hs:12:15) In the expression: (x', maxBound) :: (a, a) In a pattern binding: (x, max) = (x', maxBound) :: (a, a) In an equation for `toEnum': toEnum n = Cyc x where (x, max) = (x', maxBound) :: (a, a) x' = toEnum $ n `mod` ((fromEnum max) - 1) The problem comes up when I'm trying to give hints to the compiler about the type that a particular expression should have. My questions are: (1) what exactly is going on here, and (2) is there any general technique for specifying types in situations like this that gets around this problem?