principle of least surprise
Magnus Fröberg
mfroberg@REDACTED
Mon Nov 21 21:03:34 CET 2005
But, surprise:
-define(is_foo(X), is_atom(X) ; is_tuple(X), (size(X) == 2)).
/Magnus
Martin Bjorklund wrote:
>After the obfuscation contest we now know that parentheses are
>important in guards...
>
>I have a datatype foo which is either an atom or a tuple of size 2.
>
>It would be nice with a macro to test if a certain value is a foo,
>e.g.
>
> -define(is_foo(X), (is_atom(X) or (is_tuple(X) and (size(X) == 2)))).
>
>Then I could use this test in guards,
>
> f(X) when ?is_foo(X) -> yes;
> f(X) -> no.
>
>Isn't this reasonable? Anyone can read and understand this code.
>
>The problem is that this won't work; if I call f(foo) it will return
>no. The reason is that all expressions in my guard will be evaluated,
>and that failure in a boolean expression will fail the guard which is
>interpreted as false. (and in this case size(foo) fails).
>
>So I tried some alternatives:
>
> -define(is_foo(X), (atom(X) or (tuple(X) and (size(X) == 2)))).
>
>not that I thought that this would work, but it won't even compile.
>Why do we have atom/1 and is_atom/1???
>
>And I know that this one doesn't work.
>
> -define(is_foo(X), (is_atom(X) orelse (is_tuple(X) andalso (size(X) == 2)))).
>
>Sigh.
>
>Maybe we shouldn't be allowed to write code like this? No...
>
>My radical suggestion is:
>
> o make sure or,and etc has precedence over ==,/= etc
> (like orelse/andalso)
> o _remove_ orelse/andalso completely from the language
> (what's the probability of that?)
>
>And then I think (size(X) == 2) should be false if X is not something
>you can do size on. But that's probably out of the question.
>
>
>
>/martin
>
>
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