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anders_n
Posted: Tue Jul 04, 2006 4:09 pm Reply with quote
User Joined: 28 Feb 2005 Posts: 155 Location: Saltillo, Mexico
Hi
I recently gave an Erlang course and one of my students found
the following.
I would have expected a compiler error in this case. But as You
can see it compiles and runs with a somewhat unexpected result.
A similar error is a case statement gives a compiler error.

-module(if_bug).

-export([foo/1]).

foo(X) ->
if
X > 1 ->
ok; % <- Note semicolon here
not_ok;
true ->
true
end.


Erlang (BEAM) emulator version 5.5 [source] [async-threads:0] [hipe]

12> c(if_bug).
{ok,if_bug}
13> if_bug:foo(2).
ok
14>


/Anders
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Guest
Posted: Tue Jul 04, 2006 5:57 pm Reply with quote
Guest
Hi,

> foo(X) ->
> if
> X > 1 ->
> ok; % <- Note semicolon here
> not_ok;
> true ->
> true
> end.

This is because "not_ok; true" is a valid guard expression and guards
have higher priority in the parser than the if clause....

Can be confusing, I agree.

best regards,
Vlad
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Guest
Posted: Tue Jul 04, 2006 9:52 pm Reply with quote
Guest
Anders Nygren wrote:
>
> foo(X) ->
if
X > 1 ->
ok; % <- Note semicolon here
not_ok;
true ->
true
end.

You are being misled by the indentation.
If you write it like this, it's easier to see:

foo(X) ->
if
X > 1 ->
ok; % <- Note semicolon here
not_ok; true ->
true
end.

Semicolon within a guard separates disjunctions, i.e.,
it's an "or" operator. It can't be nested, and has
lower precedence than comma ("and").

It would have made a nice submission to the Obfuscated
Erlang Contest, though. Smile

/Richard
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