[erlang-questions] (newbie) Using Functions as Guards in Erlang

Paul Mineiro <>
Fri Jun 13 20:18:12 CEST 2008


Couldn't one implement the "evaluate into in a variable, then match"
strategy with a parse transform?

The resulting error messages might confuse (a call to foo/3 leading to a
bad match in foo/7, but there's no foo/7 explicit in the code).  However
the code would look nice.

-- p

On Fri, 13 Jun 2008, Thomas Lindgren wrote:

>
> --- Jachym Holecek <>
> wrote:
>
> > It could be useful and I admit I don't understand
> > why is that prohibited
> > (besides
> > "side effects and infinite loops in guards are Evil"
> > to which one can
> > reply "the
> > compiler can refuse a guard function if it can't
> > prove it to be safe").
>
> Arguably, that's already the case :-)
>
> > Anyway,
> > I'm not a compiler expert, so maybe there's a good
> > reason I'm missing.
>
> There are some potential compiler gains from having
> easy-to-analyze guards (e.g., you can see whether
> clauses are mutually exclusive more easily/often and
> can easily reorder tests in guards), but I think this
> restriction is mostly for historical reasons. Consider
> that you can write arbitrary "guards" using standard
> Erlang expressions, e.g.:
>
> case catch (E1 andalso E2 andalso ... E3) of
>    true -> Body;
>    _ -> <next clause>
> end
>
> or even the useful
>
> case catch abstype:is_abstype(X) of
>    true -> Body;
>    _ -> <next clause>
> end
>
> So, as far as I'm concerned, if you in practice end up
> writing your complex tests using case instead of
> guards, the guard syntax becomes less and less
> relevant. Why hang on to such a restriction when it
> just leads to more convoluted programs? Nowadays I'm
> thus in favour of allowing arbitrary guards (possibly
> disallowing side-effects by dynamic checking, but
> perhaps I'm just reluctant to let go entirely :-).
>
> Best,
> Thomas
>
> PS. The concurrent logic languages contemporary with
> early Erlang in the 1980s also required flat guards,
> but in those cases there was good reason to do so: it
> was simply too hard to implement efficient arbitrary guards.
>
>
>
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