principle of least surprise
Mon Nov 21 15:19:22 CET 2005
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,
-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)))).
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
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.
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