toString, aka overloading problems

[Alan Grover]

The problem I have is attempting to convert various structures to string
representations for printout.


Here's each piece I can't figure out. I've changed the problem from String
to something else so multiple abstract types don't get in the way.

---
Part 1

Implementations:
	class test a :: a -> [Int]

	// Pretend these instances do something useful
	instance test [Int]
		where
		test a = (map ((+) 1) a)

	instance test [[Int]]
		where
			test a = (map sum a)		// <--- line 67

Generates
	Error [test.icl,67,test ([])]: test ([]) multiply defined symbol

If the instances are alternately commented out, the derived types are
respectively:
	test ([]) :: ![Int] -> [Int];
	test ([]) :: ![[Int]] -> [Int];

If the second instance is commented out, and I try:
	bobo = test [[1]]		// <-- line 69

I get the error:
	Type error [test.icl,69,bobo]: "argument 1 of test" cannot unify
demanded type [[Int]] with [Int]
Which is kinda what I hoped for.

Since the types are distinct, [[]] vs [], I think I should be able to have
instances of them. The compiler prevents me from evaluating the wrong type,
and it derives the two types, yet seems to get it wrong in instances.

Is there a way to do this:
	define a function a -> b
		such that
			if 'a' is not a list f' is applied,
			if 'a' is a list, f'' (function a) is applied,
			where
				f' converts 'a' to 'b'
				f'' converts [b] to 'b'
	so arguments of the various types 'x', [x], [[x]], [[[x]]], etc,
will work?
--
Part 2

For the implementation:

	:: BitString = BitString	// pretend this is a useful type

	class test a :: a -> BitString

	// pretend these instances do some useful conversion
	instance test [a] | toInt a
		where test x = BitString
	instance test [a] | toReal a
		where test x = BitString	// <-- line 78

I get the error:
	Error [test.icl,78,test ([])]: test ([]) multiply defined symbol

I thought that the instances where analogous to partial functions, and I
could just specify more. Apparently not.

Is there a way to do this (use fn A if toReal works, use fn B if toInt works)?

---
"Alan Grover, Technical Pb" awgrover@umich.edu
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