[m-dev.] Default typeclass method implementations

[Fergus Henderson]

On 29-Sep-2000, Ralph Becket <rbeck at microsoft.com> wrote:
> It occurs to me (this was discussed ages ago) that it shouldn't
> be too hard to extend the current typeclass scheme to include
> default method implementations.

Yes.

> For example (inventing lots of
> syntax and assuming functional dependencies)...
> 
> :- typeclass seq(S, T) given (S >> T) where [
> 	func head(S) = T,
> 	func tail(S) = S,
> 	func cons(T, S) = S,
> 	func empty = S,
> 	func singleton(T) = S `defaults_to` [
> 		singleton(X) = cons(X, empty)
> 	],
> 	func append(S, S) = S `defaults_to` [
> 		append(Xs, Ys) =
> 			( if Xs = empty then Ys else cons(head(Xs),
> append(tail(Xs), Ys)) )
> 	],
> 	func reverse(S) = S `defaults_to` [
> 		reverse(Xs) =
> 			( if Xs = empty
> 			  then empty
> 			  else append(reverse(tail(Xs)),
> singleton(head(Xs))) )
> 	]
> ].
> 
> Any thoughts?

The syntax you've suggested above won't work well for multi-moded
methods.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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