[Haskell-cafe] Help with error

Jeff φ jeff1.61803 at gmail.com
Sat Feb 16 04:06:48 EST 2008

2008/2/15 Antoine Latter <aslatter at 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
    from_seq seq = from_seq' seq
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