[m-dev.] diff: fix bug in pprint__to_doc

[Ralph Becket]

> library/pprint.m:
> 	Fix a bug: there was an infinite loop in the code for `to_doc'.
> 	The recursive call to convert the term's arguments was
> 	happening even in the case where the term had no arguments.

This I don't understand.  The only place where a recursive call
ocurrs in the original source is in the clausal part of the
function:

:-
    deconstruct(X, Name, Arity, UnivArgs),
    Args = list__map(
        ( func(UnivArg) = to_doc(Depth - 1, univ_value(UnivArg)) ),
        UnivArgs
    ).

Now, I would (did) assume that after the call to deconstruct/4 that
(Arity = 0) <=> (UnivArgs = []), in which case the call to list__map/2
shouldn't have made any calls to to_doc/2 - if this is indeed the
error case.

Got it!  The error case is actually when Arity > 0, but Depth =< 0.
This will lead to unwanted recursive calls.

Still, unless you've somehow created a circular structure, I don't
see how this would turn into an infinite loop (surely all terms are
finite trees?)  Maybe your `infinite loop' was really just `very deep
recursion'?

Hmm.  But the new version does indeed knock this bug on the head!

--
Ralph Becket      |      MSR Cambridge      |      rbeck at microsoft.com 
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