[Haskell-cafe] bounded ranges
Chad Scherrer
chad.scherrer at gmail.com
Thu Jul 22 18:21:49 EDT 2010
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:
bdRangeSize :: (Ix i, Bounded i) => i -> Int
bdRangeSize _ = rangeSize (minBound, maxBound :: i)
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
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