[mercury-users] Incomprehensible error output, even with -E.

[Julien Fischer]

On Thu, 2 Feb 2012, Michael Richter wrote:

> On 2 February 2012 05:50, Julian Fondren <ayrnieu at gmail.com> wrote:
>
>> to make the problem clearer, you have here
>>
>>   A = (func(X) = B, C),
>>
>> How is this not like your definitions of named functions?
>>
>>
> It's got a lambda instead of a :- func body?

It has nothing to with it being a lamda.

> Maybe I'm fundamentally misunderstanding the syntax of funcs vs. preds.
> (There doesn't appear to be a grammar for Mercury in the language
> reference so I'm only inferring what's permitted and what's not.)

For the syntax of functions see the ``Rules'' section of the reference
manual (Section 2.8).  Of relevance here are the following statements
from that section.

     A function rule is an item of the form ``Head = Result :- Body''.
     The Result and the arguments of Head must be valid data-terms.
     The Body must be a valid goal.

In this instance we have ``func(X) = B, C''.  "B, C" is a goal, *not*
a data-term (i.e an expression).  That is what the initial error
message is trying to tell you:

     man_or_boy.m:030: In clause for predicate `main'/2:
     man_or_boy.m:030: � error: the language construct `,'/2 should be used as a
     man_or_boy.m:030: � goal, not as an expressn

Julien.