[Haskell-cafe] Help with error

[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

[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...@gmail.com>:

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>
>

[Jeff φ]

2008/2/15 Antoine Latter <asla...@gmail.com>:

> (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?
>

Yes, this helped. I added the type variable, e, to my From_seq class and it
worked. Thank you for the explanation. Here are the changes I made:

class From_seq t e where

from_seq :: (Sequence seq) => (seq e) -> (t e)

instance From_seq [] e where

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

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