(sent to the list this time) The problem is in the type-signature for from_seq: from_seq :: (Sequence seq) => (seq e) -> (t e) Neither the From_seq class or the type signature of the from_seq function place any restrictions on the type of e, so the type can be rewritten as: from_seq :: forall e seq . Sequence seq => (seq e) -> (t e) That is, the class explicitly defines from_seq has having norestrictions on e. Your from_seq' function requires the type e (in the error, e1) to inhabit IArray a e. The IArray constraint isn't compatible with the From_seq class definition. You may need to explore multi-parameter type classes: http://en.wikibooks.org/wiki/Haskell/Advanced_type_classes Does this help? -Antoine 2008/2/15 Jeff φ :

Hello,

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.)

{-# OPTIONS_GHC -fglasgow-exts -fbreak-on-exception -fallow-undecidable-instances #-}

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:

Error.hs:41:19: 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.

Thanks,

Jeff

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