[Haskell-cafe] Can't figure out cmap in hmatrix

Carter Schonwald carter at cs.dartmouth.edu
Mon May 30 22:33:53 CEST 2011

this is actually a bug in the type of cmap, a fix is due in the next release
(at least thats what Alberto indicated to me when I asked about this a
monthish ago) (note how you have the container type c e, but we want c a and
c b ). Instead  use the vector map or matrix map ops directly

-Carter schonwald

On Mon, May 30, 2011 at 3:27 PM, Mats Klingberg <maklingberg at gmail.com>wrote:

> Hello,
> I'm playing around a bit with the hmatrix package (
> http://hackage.haskell.org/package/hmatrix) but can't quite figure out how
> to make the cmap function in Numeric.Container work.
> An example:
> ghci> import Numeric.LinearAlgebra
> ghci> let v = fromList [1.0,2.0,3.0]
> ghci> v
> fromList [1.0,2.0,3.0] :: Data.Vector.Storable.Vector
> ghci> :t v
> v :: Vector Double
> ghci> cmap sqrt v
> <interactive>:1:1:
>    No instance for (Container Vector e0)
>      arising from a use of `cmap'
>    Possible fix: add an instance declaration for (Container Vector e0)
>    In the expression: cmap sqrt v
>    In an equation for `it': it = cmap sqrt v
> ghci> :t cmap
> cmap
>  :: (Container c e, Element b, Element a) => (a -> b) -> c a -> c b
> There is an instance for (Container Vector Double) but I assume that since
> the signature of cmap doesn't mention the type variable 'e' GHCi can't infer
> it. Googling hasn't helped me so far, except for digging up another post to
> this list with the same (?) problem, but no answer:
> http://www.haskell.org/pipermail/haskell-cafe/2011-April/091390.html
> Is there a way to tell GHC what instance to use, or how should cmap be
> used?
> Thanks!
> Mats
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