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<  Erlang  ~  illegal buard expression
jz87
Posted: Thu Jan 10, 2008 1:05 pm Reply with quote
Joined: 10 Jan 2008 Posts: 3
I wrote the following code:

Code:
if is_process_alive(Pid) -> ...


and got an illegal guard expresson error. I thought BIF are ok in guard expressions
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hao
Posted: Thu Jan 10, 2008 3:23 pm Reply with quote
User Joined: 20 Aug 2007 Posts: 18 Location: Uppsala, Sweden
It seems that BIF is_process_alive/1 is not allowed in guard tests for "if" control statement as well as for a function.

Code:

t(Pid) ->
    Flag = is_process_alive(Pid),
    if Flag == true ->
            io:format("In t/1: the given process is alive.~n");
       true ->
            io:format("In t/1: the given process is NOT alive.~n")
    end,
    ok.

t/1 works well.

Code:

t1(Pid) when is_process_alive(Pid) ->
    io:format("In t1/1: the given process is alive.~n"),
    ok;
t1(_Pid) ->
    io:format("In t1/1: the given process is NOT alive.~n"),
    ok.

t1/1 compilation fails and gets illegal guard expression exception.

Code:

t2(Pid) when is_process_alive(Pid) == true ->
    io:format("In t2/1: the given process is alive.~n"),
    ok;
t2(_Pid) ->
    io:format("In t2/1: the given process is NOT alive.~n"),
    ok.

t2/1 compilation fails and gets illegal guard expression exception as well.
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bluefly
Posted: Thu Jan 10, 2008 7:26 pm Reply with quote
User Joined: 06 Jan 2008 Posts: 10
I just checked the man for it ("erl -man erlang" or http://www.erlang.org/doc/man/erlang.html), and it looks like is_process_alive/1 is one of many BIFs that do not have the "Allowed in guard tests" line.

I remember reading in a tutorial somewhere that the BIFs are generally allowed... ah, I was wrong. I found this line:

"Only a few built in functions can be used in guards, and you cannot use functions you have defined yourself in guards."

http://erlang.org/doc/getting_started/seq_prog.html#2.12
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hao
Posted: Fri Jan 11, 2008 10:11 am Reply with quote
User Joined: 20 Aug 2007 Posts: 18 Location: Uppsala, Sweden
What bluefly mentioned is right. We can find out all about the guard tests in the 6.24 Guard Sequences in the Erlang Reference Manual. (http://erlang.org/doc/reference_manual/expressions.html) Wink
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