[erlang-questions] Reading, Learning, Confused

Edwin Fine <>
Sat Jul 19 18:11:26 CEST 2008


I interpret those words as follows:

As of Erlang 5.5/OTP R11B, short-circuit boolean expressions are allowed in
guards. Please note that evaluation is always short-circuited in guards.
This is because guard tests are known to be free of side effects. If a guard
condition is free of side-effects, that means that in a sequence of guards,
it is guaranteed that leaving out the evaluation of one or more guards will
not change the state of the program. For example, let's say that
user-defined functions were allowed in guards.

f(X) -> put(x, X), true.
g() -> put(x, true), put(y,"g() was called"), true.
h() -> get(x).
test(X) when f(X), g(), h() ->  ok. % Will not compile - illegal Erlang

What will get(x) and get(y) return after test(X) is run? Will it make a
difference to the result if guards are not short-circuited?

Non-short-circuited guards:
test(true): f(true) sets x to true, g() sets x to true, y to the string. End
result: x is true, y is "g() was called".
test(false): f(false) sets x to false, g() sets x to true. End result: x is
true, y is "g() was called".

Short-circuited guards with side-effects:
test(true): f(true) sets x to true, g() sets x to true, y to the string. End
result: x is true, y is "g() was called".
test(false): f(false) sets x to false, g() is not evaluated. End result: x
is false, y is undefined.

So skipping a guard test if it has a side effect can change the state of the
whole program, which would make short-circuit evaluation impossible. But
because no guard tests can have side-effects, the state of the program
cannot be changed if one or more guard tests are not evaluated.

Hope this makes sense.



On Sat, Jul 19, 2008 at 11:22 AM, Alpár Jüttner <> wrote:

> >         As of Erlang 5.5/OTP R11B, short-circuit boolean expressions are
> >         allowed in guards. In guards, however, evaluation is always
> >         short-circuited since guard tests are known to be free of side
> >         effects.
> >         (Section 6.14, Short-Circuit Boolean Expressions)
> >
> > Something is wrong here, isn;t it?
>
> I mean
>
>      * What does the word "however" mean here? Does it mean that if
>        they are not in a guard, orelse/andelse might be non
>        short-circuited?
>      * How does the freedom from side effects are related to the
>        short-circuited evaluation?
>
> Regards,
> Alpar
>
> >
> > Regards,
> > Alpar
> >
> > On Sat, 2008-07-19 at 06:50 -0700, Lev Walkin wrote:
> > > Sean Allen wrote:
> > > > by a small bit of example code in Programming Erlang related to
> guards
> > > > and short circuit booleans:
> > > >
> > > >   f(X) when (X == 0) or (1/X > 2) ->
> > > >      ...
> > > >
> > > > g(X) when (X == 0) orelse ( 1/X > 2) ->
> > > >     ...
> > > >
> > > > The guard in f(X) fails when X is zero but succeeds in g(X)
> > > >
> > > > Can someone explain why?
> > >
> > >
> > > Sean,
> > >
> > > The thing is, "or" does not short-circuit evaluation when left side
> > > succeeds, whereas "orelse" does. Same short-circuit logic is
> > > behind the differences between "and" and "andalso".
> > >
> > > Actually, the very book you read explains these differences and warns
> > > about caveats a couple pages later (or earlier). Don't stop reading.
> > >
> >
> > _______________________________________________
> > erlang-questions mailing list
> > 
> > http://www.erlang.org/mailman/listinfo/erlang-questions
>
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> 
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>



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