[Haskell-cafe] Help with error

Jeff φ jeff1.61803 at gmail.com
Fri Feb 15 21:47:23 EST 2008


I get an error message on the code below with GHC.  I can't figure out how
to get rid of the error.  I'd appreciate suggestions on how to fix this.
(BTW, the code may look overly combersome because I stripped out anything
unnecessary to demonstrate the error.)


import qualified Prelude
import Prelude
import Data.Array.IArray

class Sequence seq where
    slength :: (seq e) -> Int
    snull :: (seq e) -> Bool
    shead :: (seq e) -> e
    stail :: (seq e) -> (seq e)

instance Sequence [] where
    slength = length
    snull = null
    shead = head
    stail = tail

class From_seq t where
    from_seq :: (Sequence seq) => (seq e) -> (t e)

instance From_seq [] where
    from_seq seq
    | snull seq  = []
    | otherwise = (shead seq) : (from_seq (stail seq))

from_seq' seq
    | snull seq = (listArray (0,-1) [])
    | otherwise = listArray (0,fromIntegral (slength seq) -1) (from_seq seq)

When I uncomment this out, I get the error messages:

    Could not deduce (IArray a e)
      from the context (From_seq (a i),
                        Ix i,
                        Num i,
                        IArray a e1,
                        Sequence seq)
      arising from a use of `from_seq'' at Error.hs:41:19-31

instance (Ix i, Num i, IArray a e) => From_seq (a i) where
    from_seq seq = from_seq' seq

When I load the module above, I can evaluate the folloing in GHCI

*Main> from_seq' [0..5] :: Array Int Double
array (0,5) [(0,0.0),(1,1.0),(2,2.0),(3,3.0),(4,4.0),(5,5.0)]

But, I'd like to do this with the From_seq class.  If anyone has
suggestions, I'd be grateful.


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