[Haskell] Help in understanding a type error involving forall
and class constraints
duncan.coutts at worcester.oxford.ac.uk
Tue Jun 29 18:18:54 EDT 2004
On Tue, 2004-06-29 at 18:42, MR K P SCHUPKE wrote:
> Erm, something I remember about needing MArrays... Here's something
> which does the same thing (cooks and MArray then freezes it - also using
> STUArray... The neat thing is MArray is a class, so you can swap between
> STUArray and STArray implementations without changing code.
I tried generalising the type to not mention which of STUArray and
STArray I'm using. Letting ghc infer the type gives this type,
unfortunately annotating the function with this type gives a type
error!! Compiler/typechecker bug perhaps?
buildUArray bounds f = do arr <- buildUArray' bounds f
ghc infers this type:
buildUArray :: forall a i b s array.
(MArray (array s) a (ST s), Ix i, IArray b a) =>
(i, i) -> (i -> a) -> ST s (b i a)
however if one add this annotation I get the same error message I got
Could not deduce (MArray (array1 s) a (ST s))
from the context (MArray (array s) a (ST s), Ix i, IArray b a)
arising from use of `unsafeFreeze' at BuildArray.hs:25
Add (MArray (array1 s) a (ST s))
to the type signature(s) for `buildUArray'
Or add an instance declaration for (MArray (array1 s) a (ST s))
In the result of a 'do' expression: unsafeFreeze arr
In the definition of `buildUArray':
buildUArray bounds f
arr <- buildUArray' bounds f
> This is the classic dynamic programming version of string difference:
The difference with your example cod is that I return the array itself
rather than a value calculated using the array. With your code, as soon
as we try to return the array too, I run into the same problem.
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