[erlang-questions] illegal guard expression for IF illegal guard expression for "if"

[Ulf Wiger]

You should see guards as an extension of pattern matching.

If a guard function would have side-effects, the overall evaluation would most likely not be deterministic, and - most importantly perhaps - would be extremely difficult to understand and reason about. Ideally, the function should also be O(1), although a few O(N) guards, like length/1, are also allowed. For this reason, programmers can normally regard pattern-matching as extremely efficient, and use it extensively, trusting that the compiler will do a good job optimizing it.

It is correct that the compiler is not able to determine, at present, whether a function is referentially transparent. It also cannot determine the complexity of a function.

BR,
Ulf W

On 4 Dec 2011, at 09:51, Barco You wrote:

> But, if explaining in this way it has nothing to do with side-effect. It's just because the guard expression is stochastic rather than deterministic.
> 
> If having a deterministic function in the 'if' guard expression, we still get 'illegal guard expression'. According to the document provided by some guys in previous mails, the foundamental reason is that the erlang VM does not make sure whether a function in the guard expression has side-effect.
> 
> So, I hope to know why functions with side-effect can not be the guard expression.
> 
> Thank you all!
> Barco
> 
> On Dec 2, 2011 10:42 PM, "Barco You" <barcojie@REDACTED> wrote:
> Ooooh! Maybe I understand now!
> 
> It means that if the first branch is not satisfied, saying get 0.7, then random:uniform() would be evaluated again in the second branch and it possible to get a value less than 0.5, which then makes a paradox. Right?
> 
> In the contrast, with case statement, the condition is only evaluated once no matter whichever branch would be satisfied.
> 
> How smart erlang is, and how amazing you erlangers are!
> 
> Barco
> 
> On Dec 2, 2011 6:18 PM, "CGS" <cgsmcmlxxv@REDACTED> wrote:
> Just to make it clearer:
> 
> The second condition is equivalent with the expression: "if the value of random:uniform() is greater or equal to 0.5 then report 'bad'". In this case, if you entered with value 0.4 in the first branch and modified it to 0.7 in the first branch, the second branch should be executed as well. That is against the if statement purpose which should report only one branch at the time as valid. Therefore, Erlang requires for the compared values to be constant during the if statement processing.
> 
> CGS
> 
> 
> 
> On 12/02/2011 11:06 AM, CGS wrote:
>> 
>> If statement is not a block statement (to fix the variables values at the entrance of the statement) in Erlang. That means, if an uniform:random() value (say, 0.4) is used for checking the first branch condition and the value is changed within the branch (say, 0.7), then you enter a paradox related to if the function should be considered for the first or the second branch. To avoid this, Erlang requires an assurance that the compared values do not change while processing the if statement.
>> 
>> CGS
>> 
>> 
>> 
>> On 12/02/2011 10:56 AM, Barco You wrote:
>>> 
>>> Hi CGS,
>>> 
>>> I can't understand your statement --- " if in the first branch you use again random:uniform(), the second branch condition can report an inaccurate result."  Could you please make it clearer? Thanks!
>>> 
>>> Hi Others,
>>> 
>>> Why functions with side effect can not be in the guard expressions?
>>> 
>>> 
>>> Thank you!
>>> Barco
>>> 
>>> On Fri, Dec 2, 2011 at 5:50 PM, CGS <cgsmcmlxxv@REDACTED> wrote:
>>> Hi,
>>> 
>>> To put in simple words for better understanding, you can have only constant variables withing the guard expression. That means, in your case, if in the first branch you use again random:uniform(), the second branch condition can report an inaccurate result.
>>> 
>>> Alternatively, you can use case statement:
>>> 
>>> case (random:uniform()<0.5) of
>>>      true -> good;
>>>      false -> bad
>>> end
>>> 
>>> I hope this answer will help you.
>>> 
>>> CGS
>>> 
>>> 
>>> 
>>> 
>>> On 12/02/2011 10:30 AM, Barco You wrote:
>>>> Why does the following expression got "illegal guard expression" when compiling:
>>>> X = 0.5,
>>>> if
>>>>     random:uniform() < X ->      %error reported for this line
>>>>            good;
>>>>     true ->
>>>>            bad
>>>> end.
>>>> 
>>>> But if I change it to following expression, it's ok:
>>>> X = 0.5,
>>>> Ran = random:uniform(),
>>>> if
>>>>     Ran < X -> 
>>>>            good;
>>>>     true ->
>>>>            bad
>>>> end.
>>>> 
>>>> BRs,
>>>> Barco
>>>> 
>>>> _______________________________________________
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>>> 
>>> 
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>> 
> 
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