principle of least surprise
Wed Nov 23 00:31:02 CET 2005
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,
> -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).
Are you sure that this is the reason? When boolean guard expressions
were added we defined a strict left to right evaluation jst so we could
handle cases like this. One of my standard test cases was something like:
when is_atom(X) or (is_integer(X) and (X + 5 < 6)) ->
(test cases are seldom reasonable). It worked then. Has the compiler
been changed since then?
>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
> (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.
I quite honestly don't really see the problem with the precedences.
Whatever is wrong with your macro it isn't the precedences. How do you
envisage that (size(X) == 2) be false? What should size of something
Since and/or behave as "normal" operators it was thought better to add
new constructions with explicit left-to-right semantics rather than
modify and/or. After a while you get into a horrible mess if you start
special casing certain functions (operators are functions).
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