There are situations where it is important that guards are fast and side effect-free (in receive, for example).
Conditions in if-expressions is *not* one of those places, IMHO. (Except for the usual good reasons to
to program as 'functionally' as possible).
Ie, there is no reason that I can see why the if-condition has to be a guard syntactically rather than an expression.
I just forces the programmer to write something more clumsy.
Or is there some light I haven't yet seen?
-- Thomas
-----Original Message-----
From: owner-erlang-questions@REDACTED
[mailto:owner-erlang-questions@REDACTED]On Behalf Of Matthias Lang
Sent: den 17 maj 2005 13:20
To: Dietmar Schaefer
Cc: erlang-questions@REDACTED
Subject: Re: newbie question
>From the FAQ:
| 9.4. Why can't I call arbitrary functions in a guard?
|
| If that was allowed, there would be no guarantee that guards were side-effect free.
|
| Also, it is convenient to be able to program as though guards do not consume any significant amount of execution time. There's a list of BIFs which can be called from within guards in the Erlang book and the standard Erlang spec, some examples are size(), length(), integer(), record().
|
| The "problem" often crops up when using if:
|
| issue_warning() ->
| if (os:type() == {win32, windows}) -> %% illegal guard
| ok = io:fwrite("this computer may crash at any time\n");
| true ->
| ok = io:fwrite("no problem\n")
| end.
|
|
| The solution is usually to use case instead. Case is used much more frequently than if in most Erlang programs:
|
| issue_warning() ->
| case os:type() of
| {win32, windows} -> ok = io:fwrite("crash soon\n");
| _ -> ok = io:fwrite("no problem\n")
| end.
|
Matthias
Dietmar Schaefer writes:
> Hi !
>
> Isn't that supposed to work ?
>
>
> if cmmc_db:getProcState(Key,Name) /= State ->
>
> instead I have to write
>
> ProcState = cmmc_db:getProcState(Key,Name),
>
> if ProcState /= State ->
>
>
>
> regards
>
>
> Dietmar