# Old style vs. new style boolean expressions
Håkan Stenholm
<>
Tue Oct 8 21:57:16 CEST 2002
>Here is a function written in classic Erlang style:
>
>is_alphanum(N) when N >= $A, N =< $Z -> true;
>is_alphanum(N) when N >= $a, N =< $z -> true;
>is_alphanum(N) when N >= $0, N =< $9 -> true;
>is_alphanum($_) -> true;
>is_alphanum(_) -> false.
In R8 this can also be written as:
>is_alphanum(N) when N >= $A and N =< $Z -> true;
>is_alphanum(N) when N >= $a and N =< $z -> true;
>is_alphanum(N) when N >= $0 and N =< $9 -> true;
>is_alphanum($_) -> true;
>is_alphanum(_) -> false.
>
>This can also be written using boolean expression guards, which were
>introduced in (I think) R8:
>
>is_alphanum(N) ->
> ((N >= $A) and (N =< $Z)) or
> ((N >= $a) and (N =< $z)) or
> ((N >= $0) and (N =< $9)) or
> (N == $_).
Im fairly sure this has been available since R5 (possibly undocumented),
the 'andalso' and 'orelse' below on the other hand are from R8.
>
>This isn't as efficient as the first method, because the boolean
expression
>in the second code snippet is always fully evaluated. To fix this, use
>"andalso" and "orelse":
>
>is_alphanum(N) ->
> ((N >= $A) andalso (N =< $Z)) orelse
> ((N >= $a) andalso (N =< $z)) orelse
> ((N >= $0) andalso (N =< $9)) orelse
> (N == $_).
>
>Now the point: The first vesion generates by far the smallest beam file,
>the second is somewhat larger, and the third is still larger. When
fiddling
>with a module containing many similar functions, switching from first
to the
>third method increased the beam file size by 40%.
I don't think this make much of a difference most of the time, and even
if it
increases the size noticeably - does it matter ?
>
>Is this just because boolean expressions are a relatively new
addition, and
>so the R8 compiler doesn't deal with them well, or is it deeper than that?
>
>I'm still not sure whether I think the first or third method is cleaner :)
I think I like the first version is best (in this case), it's fairly clear
that N is checked to see if it belongs to a certain char range as well
as what
is returned if the match is true/false.
I think the main difference is that the first version only requires you to
comprehend the meaning of one simple match at a time, while the later
versions
require you to comprehend one big boolean expression all at once.
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