Two problems

[Lennart Augustsson <augustss>]

Original-Via: uk.ac.ukc; Sat, 26 Jan 91 17:58:41 GMT
From: Lennart Augustsson <augustss>
Date: Sat, 26 Jan 91 18:52:49 +0100
To: haskell@cs.glasgow.ac.uk
Subject: Two problems
Original-Sender: augustss%se.chalmers.cs@sunic.sunet.se
Sender: haskell-request@cs.glasgow.ac.uk


Two problems:
-------------

I wrote the following little program the other day
	instance (Num a) => Num (a,a) where
		(a,b) * (c,d) = (a*c+a*d+b*c, a*c+b*d)
	fib n = snd ((1,0)^n)
and immediately ran into two problems, one was anticipated
and the other was not.

1. It's not allowed to make an instance declaration in a module
that does not contain the type and/or class declaration of the
involved type and class.  Why is this so?  Why can't I define
how pairs should be multiplied locally in a module?
This restriction forces me to write
	type TPair a b = Pair a b
	instance (Num a) => Num (Pair a a) where
		(Pair a b) * (Pair c d) = Pair (a*c+a*d+b*c) (a*c+b*d)
	psnd (Pair x y) = y
	fib n = psnd ((Pair 1 0)^n)

This problem was anticipated (and the restriction has in fact been
removed in Haskell B).

2. My second problem came as a surprise and I don't know what the answer
is.  This is the real reason for this posting and I hope that the
knowledgable Haskell commitee can give me the answer.
The typing of "(1,0)" (I will use the builtin pairs for readablility) is
"(1,0) :: (Num a, Num b)=>(a,b)".  The instance declaration only says
that "Num a => (a,a)" is an instance.  Does this mean that the program has
a type error or should the compiler conclude that the 0 and 1 must have
the same type?  The Haskell B compiler complains about a type error.
The Glasgow Haskell compiler (version 0.402) does not complain, but the
executable file it produces causes a core dump.

What's the right behaviour?

	-- Lennart Augustsson