[Haskell-beginners] Errors involving rigid skolem types
Matthew Moppett
matthewmoppett at gmail.com
Mon Aug 27 16:04:37 CEST 2012
A couple of clarifications regarding my previous post:
It should be ((fromEnum max) + 1) rather than ((fromEnum max) - 1); and
Perhaps question (2) should be: are there any lessons to be learnt about
how to avoid this problem in future?
On Mon, Aug 27, 2012 at 11:21 PM, Matthew Moppett
<matthewmoppett at gmail.com>wrote:
> 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?
>
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