[erlang-questions] Dict size in guards

[Richard O'Keefe]

On 11/02/2011, at 5:05 AM, Alessandro Sivieri wrote:

> Hi all,
> 
> is there a good reason for not allowing dict:size() in guards?

Yes.  Who knows what dict:size() actually does?
Guards cannot and should not call functions.

> I mean, it
> looks exactly like length() for lists, in theory (of course, their
> implementations are different).

The inclusion of length/1 in guard tests has always been more than
a bit dodgy.  It's better taste to avoid it.  Not least because
if you do something like

f(L)
   when length(L) > 10 -> long_case(L);
   when length(L) >  5 -> medium_case(L);
   when true           -> short_case(L).

doesn't do what people often think.  I've several times been
caught by that.

f(L) ->
    N = length(L),
    if N > 10 -> long_case(L)
     ; N >  5 -> medium_case(L)
     ; true   -> short_case(L)
    end.

is safer and only evaluates the length once.

A style warning "length/1 in guard" would be nice to have.