Dear sir,
I had written several pages of Haskell scripts, tried HUGS Haskell
on them, debugged, and now I'm trying to complile the stuff with
Glasgow Haskell.
ghc-0.25 dislikes the following declarations:
-------------------------------------
module M where
instance (Num a, Num b) => Num (a,b)
where
(x,y)+(x1,y1) = (x+x1, y+y1)
;
-------------------------------------
This module only tries to export the instance for (,):
(Num a, Num b) => Num (a,b)
HUGS Haskell (it ignores modularity) makes this (+) for pairs
all right, so one can evalute (1,1)+(1,2) -> (2,3).
But ghc-0.25
says that this instance for Num should be declared in the same
module where the type constructor is defined.
I guess, it is the (,) constructor from the Prelude module.
Should one add an instance of Num for (,) to Prelude ?
And how to do this ?
Haskell-1.3 report contains an example:
--------------
module A where
import Prelude hiding (map)
map f x = x f
--------------
This module redefines `map' totally.
Should I redefine totally (,) with its instances in this way ?
Besides, import Prelude hiding ( (,) )
is qualified as illegal syntax.
Does this all mean that to overload (+) for pairs one should
define some new constructor, isomorphic to (,) but with the
additional instances ? This does not look nice ...
Thanks in advance.
Sergei D.Meshveliani
