[R] Question about expression parser for "return" statement

[Duncan Murdoch]

On 13/11/2016 6:47 AM, Duncan Murdoch wrote:
> On 13/11/2016 12:50 AM, Dave DeBarr wrote:
>> I've noticed that if I don't include parentheses around the intended return
>> value for the "return" statement, R will assume the first parenthetical
>> expression is the intended return value ... even if that parenthetical
>> expression is only part of a larger expression.
>>
>> Is this intentional?
>
> Yes, return is just a function call that has side effects.  As far as
> the parser is concerned,
>
> return ((1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance))
>
> is basically the same as
>
> f((1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance))

By the way, out of curiosity I took a look at the source of CRAN 
packages to see if this actually occurs.  It turns out that "return" is 
used as a variable name often enough to make automatic tests tricky, so 
I don't know the answer to my question.  However, I did turn up a number 
of cases where people have code like this:

     if (name == "") return;

(from the bio.infer package), which never calls return(), so doesn't 
actually do what the author likely intended.

Duncan Murdoch

>
> Duncan Murdoch
>
>>
>> I'm guessing it is intentional; but since there is no warning about
>> ignoring the rest of the expression, it could lead to hard-to-find bugs.
>>
>> Thanks,
>> Dave
>>
>> Here's an example ...
>>
>> dnorm(2, 0, 1)
>> normalDensityFunction = function(x, Mean, Variance) {
>>     # no parentheses surrounding the entire "return" value
>>     return (1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance)
>> }
>> normalDensityFunction(2, 0, 1)    # incorrect answer
>> normalDensityFunction = function(x, Mean, Variance) {
>>     # parentheses surrounding the entire "return" value
>>     return ((1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance))
>> }
>> normalDensityFunction(2, 0, 1)    # correct answer
>>
>> 	[[alternative HTML version deleted]]
>>
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>