[mercury-users] Sneaky pseudo-buglet with typeclasses.

[Ralph Becket]

When I try to compile the following

	:- module singlearg.

	:- interface.

	:- typeclass foo(T) where [
        pred new(T),
        mode new(out) is det
	].

	:- implementation.

	:- type bar ---> bar(string).

	:- instance foo(bar) where [
        pred (new/1) is bar_new
	].

	bar_new(bar("Hi!")).

I get...

singlearg.m:015: Error: expected `pred(<Name> / <Arity>) is <InstanceName>':
pre
d new / 1 is bar_new.
singlearg.m:018: Error: clause for predicate `singlearg:bar_new/1'
singlearg.m:018:   without preceding `pred' declaration.
singlearg.m:018: Inferred :- pred bar_new((singlearg:bar)).
singlearg.m:018: Error: no mode declaration for predicate
`singlearg:bar_new/1'.
For more information, try recompiling with `-E'.

The cause of this hard-to-spot error is that I've accidentally inserted a
space between `pred' and the `(new/1)' in the instance declaration.  This
is quite a nasty one because the norm in the rest of the language is for
`pred' and `func' to stand as prefix operators - you have to get rid of the
space here or the structure parses differently and the compiler gets upset.
Beware.

To the developers: should I report this as an unclear error message?  It's
just taken me 40mins to track down and I hate this sort of 1-char bug.

Cheers,

Ralph
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