[m-dev.] bad error message

[Peter Schachte]

Probably a known flaw, but you guys asked for complaints about
bad compiler error messages....

I got this error message:

    Error: expected type in instance declaration to be a functor with
    variables as args: _1.

from this declaration:

    :- instance ground_watchable(dvar(T),T).

Firstly, note there is no line number in this error message, so the
only way I know that's the code that caused the error is that I've
only got 2 instance declarations, and I get 2 identical error
messages.  If I had more, it'd be difficult to find the problem.
Secondly, I infer that the _1 means the first argument is the one with
the error, but that one *is* a functor with variables as arguments.
Thirdly, the message doesn't make it clear you can never have type
variables in instance declarations, just that it didn't expect it in
that context.

-- 
Peter Schachte              Computers are useless. They can only give you
schachte at cs.mu.OZ.AU        answers.
www.cs.mu.oz.au/~schachte/      -- Pablo Picasso 
Phone: +61 3 8344 1338      
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