[Haskell-cafe] Help with error

[Antoine Latter]

(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 <jeff1.61803 at gmail.com>:
> 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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