id	summary	reporter	owner	description	type	status	priority	milestone	version	resolution	keywords	cc
76	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"	defect	closed	major			wontfix