Re: [Haskell-cafe] Help with error

2008-02-16 Thread Jeff φ 
2008/2/15 Antoine Latter [EMAIL PROTECTED]:

 (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
from_seq seq = from_seq' seq
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[Haskell-cafe] Help with error

2008-02-15 Thread 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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Re: [Haskell-cafe] Help with error

2008-02-15 Thread 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 [EMAIL PROTECTED]:
 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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