How overload operator in Haskell?

[Martin Sjögren]

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fre 2003-07-11 klockan 08.07 skrev Ashley Yakeley:
> In article <16140.55249.147965.603167@cerise.nosuchdomain.co.uk>,
>  Glynn Clements <glynn.clements@virgin.net> wrote:
>=20
> > > instance Num Vector where
> >=20
> > Except that class instances have to be algebraic datatypes ("data") or
> > renamed datatypes ("newtype"), but not type synonyms ("type").
>=20
> That's not true, is it? I mean as long as there isn't already some=20
> overlapping instance, I think it's quite OK.

Well, there's the "The instance type must be of form (T a b c) where T
is not a synonym, and a,b,c are distinct type variables" restriction
(ghc), or as hugs put it: "Cannot use type synonym in instance head"

You get around it with hugs -98 or ghc -fglasgow-exts. Dunno about
nhc98.


/Martin

NB: I just used  class Foo ; instance Foo String  to produce these error
messages...
--=20
Martin Sj=F6gren
  martin@strakt.com
  Phone: +46 (0)31 7490880       Cell: +46 (0)739 169191
  GPG key: http://www.strakt.com/~martin/gpg.html

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