The following code is intended as a first step towards creating a cyclical enumerable type, such that:<div> (e.g.) [Cyc Friday .. Cyc Tuesday] would yield [Friday, Saturday, Sunday, Monday, Tuesday]</div><div><br></div>
<div><div>module Cycle where</div><div><br></div><div>newtype Cyc a = Cyc a deriving (Eq, Ord, Bounded, Show, Read)</div><div><br></div><div>fromCyc :: Cyc a -> a</div><div>fromCyc (Cyc a) = a</div><div><br></div><div>
instance (Enum a, Bounded a) => Enum (Cyc a) where</div><div> fromEnum = fromEnum . fromCyc</div><div> toEnum n = Cyc x</div><div> where (x, max) = (x', maxBound) :: (a, a)</div><div> x' = toEnum $ n `mod` ((fromEnum max) - 1)</div>
</div><div><br></div><div>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:</div><div><br></div><div>
<div>Couldn't match type `a0' with `a1'</div><div> because type variable `a1' would escape its scope</div><div> This (rigid, skolem) type variable is bound by</div><div> an expression type signature: (a1, a1)</div>
<div> The following variables have types that mention a0</div><div> x' :: a0 (bound at Cycle.hs:12:15)</div><div> In the expression: (x', maxBound) :: (a, a)</div><div> In a pattern binding: (x, max) = (x', maxBound) :: (a, a)</div>
<div> In an equation for `toEnum':</div><div> toEnum n</div><div> = Cyc x</div><div> where</div><div> (x, max) = (x', maxBound) :: (a, a)</div><div> x' = toEnum $ n `mod` ((fromEnum max) - 1)</div>
</div><div><br></div><div>The problem comes up when I'm trying to give hints to the compiler about the type that a particular expression should have. </div><div><br></div><div>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?</div>