[Haskell-cafe] bounded ranges

[Daniel Fischer]

On Friday 23 July 2010 00:21:49, Chad Scherrer wrote:
> Hello cafe,
>
> I'm trying to do some things with bounded indices so I can carry
> around arrays (well, Vectors, really) without needing to refer to the
> bounds.
>
> For example, if I know my indices are Bool values, I can do
>
> > rangeSize (minBound, maxBound :: Bool)
>
> 2
>
> I'd like to be able to do this in general, but...
>
> > :t rangeSize (minBound, maxBound)
>
> <interactive>:1:11:
>     Ambiguous type variable `a' in the constraints:
>       `Bounded a'
>         arising from a use of `minBound' at <interactive>:1:11-18
>       `Ix a' arising from a use of `rangeSize' at <interactive>:1:0-29
>     Probable fix: add a type signature that fixes these type variable(s)
>
> I thought it might help to put it into a module and do a better job
> with the type, like this:
>

{-# LANGUAGE ScopedTypeVariables #-}

bdRangeSize :: forall i. (Ix i, Bounded i) => i -> Int

> bdRangeSize :: (Ix i, Bounded i) => i -> Int
> bdRangeSize _ = rangeSize (minBound, maxBound :: i)
>

or, H98, without ScopedTypeVariables and forall,

bdRangeSize x = rangeSize (minBound `asTypeOf` x, maxBound)

> but I still have problems:
>
> MyArray.hs:22:36:
>     Could not deduce (Bounded i1) from the context ()
>       arising from a use of `maxBound' at MyArray.hs:22:36-43
>     Possible fix:
>       add (Bounded i1) to the context of an expression type signature
>     In the expression: maxBound :: i
>     In the first argument of `rangeSize', namely
>         `(minBound, maxBound :: i)'
>     In the expression: rangeSize (minBound, maxBound :: i)
>
> I thought maybe it's an existential types problem or something, but I
> don't understand why it would be coming up here. Any thoughts?
>
> Oh yes, and I'm using  GHC version 6.12.1.
>
> Thanks,
> Chad