Vector cons typeclass error

[mcandre]

I'm writing a function to convert a flat vector of pixel values to a vector of RGB tuples. It will have the type:

import qualified Data.Vector.Storable as V

flat2rgb :: V.Vector Word8 -> V.Vector (Word8, Word8, Word8)

For some reason, Haskell reports a typeclass error, which I read as a failure to implement Vector.Generic.cons for all types.

$ make
ghc --make fuzz.hs -O2 -fforce-recomp -optl"-Wl,-no_compact_unwind" -package JuicyPixels -package filepath -package random-fu -package vector -package base
[1 of 1] Compiling Main             ( fuzz.hs, fuzz.o )

fuzz.hs:24:23:
    No instance for (V.Storable (Word8, Word8, Word8))
      arising from a use of `V.cons'
    Possible fix:
      add an instance declaration for (V.Storable (Word8, Word8, Word8))
    In the expression: V.cons (r, g, b) v'
    In an equation for `flat2rgb':
        flat2rgb v
          | V.length v < 3 = V.empty
          | otherwise = V.cons (r, g, b) v'
          where
              r = V.head v
              g = V.head $ V.drop 1 v
              b = V.head $ V.drop 2 v
              v' = flat2rgb $ V.drop 3 v

Or maybe I'm doing something wrong.

Specs:

• vector 0.9.1
• cabal-install 0.10.2
• Cabal library 1.10.1.0
• Haskell 7.0.3

[mcandre]

Ah, importing Foreign.Storable.Tuple from the storable-tuple package fixes the problem.

http://hackage.haskell.org/package/storable-tuple

Consider requiring storable-tuple as a dependency so that vectors can automatically work over tuples?

[rl]

I think it's preferable for the client to depend on both vector and storable-tuple if that's what it needs.