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
On Thu, 22 Jul 2010, 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:
bdRangeSize :: (Ix i, Bounded i) => i -> Int bdRangeSize _ = rangeSize (minBound, maxBound :: i)
bdRangeSize x = rangeSize (minBound, maxBound `asTypeOf` x)
On Thu, 22 Jul 2010, Chad Scherrer wrote:
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)
Henning Thielemann
bdRangeSize x = rangeSize (minBound, maxBound `asTypeOf` x)
Perfect, thank you! Chad
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
An alternative to the `asTypeOf` idiom, which is also Haskell 98, is to use a newtype instead of a dummy argument: newtype RangeSize i = RangeSize { getRangeSize :: Int } boundedRangeSize :: Ix i => (i,i) -> RangeSize i boundedRangeSize = RangeSize . rangeSize bdRangeSize :: (Ix i, Bounded i) => RangeSize i bdRangeSize = boundedRangeSize (minBound, maxBound) Now you can call ghci> getRangeSize (bdRangeSize :: RangeSize Bool) 2 without language extensions and avoid the extra applications due to the dummy argument and the `asTypeOf` call. Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)
On Jul 23, 2010, at 12:21 AM, Chad Scherrer wrote:
bdRangeSize :: (Ix i, Bounded i) => i -> Int bdRangeSize _ = rangeSize (minBound, maxBound :: i)
Unlike intended, the `i` in the type annotation to `maxBound` is not the same as the `i` in the signature of `bdRangeSize`. You need to enable ScopedTypeVariables and explicitly quantify the type variable `i` in order to refer to it in the annotation of `maxBound`: {-# LANGUAGE ScopedTypeVariables #-} import Data.Ix bdRangeSize :: forall i . (Ix i, Bounded i) => i -> Int bdRangeSize _ = rangeSize (minBound, maxBound :: i) Cheers, Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)
participants (4)
-
Chad Scherrer -
Daniel Fischer -
Henning Thielemann -
Sebastian Fischer