# principle of least surprise

mbj <>
Mon Nov 21 21:55:29 CET 2005

```Magnus Fröberg <> wrote:
> But, surprise:
>
> -define(is_foo(X), is_atom(X) ; is_tuple(X), (size(X) == 2)).

Right, but then you can't use is_foo as other booleans, e.g.

IsBar = (?is_foo(X) or is_list(X)),

or whatever.

/martin

>
> /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...
> >
> >
> >  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
> >
> >
>

```