[Rd] "if" function in pure R?

[Stepan]

Hello Alexandre,

there are two sides of your question it seems to me:

- there is no possibility to extend the R parser with new syntax. The R 
parser knows to internally "rewrite"(*) things such as

if (a>b) print(1) else print(2)

into

`if`(a>b, print(1), print(2))

The parser has a fixed set of functions that get some special treatment 
like this.

- there is possibility to implement control flow constructs thanks to 
the lazy evaluation of arguments. An example of such function is ifelse 
from base.

Best,
Stepan

(*) "rewrite" is illustrative, the exact internal working of this is not 
important for this discussion


On 27/05/2019 08:16, Alexandre Courtiol wrote:
> Thanks a lot Jiefei,
> 
> I had thought of defining a binary operator (inspired by pipes) or simply
> using an additional condition in the if() calls [e.g. if(foo & fn(bar))
> doSomeThing; with fn(bar) returning a logical], but both are workaround
> that I do not find as elegant as a proper control-flow construct.
> 
> Thus two questions remain:
> - is it possible to create a control-flow construct in pure R?
> - if yes, how?
> 
> Anyone with more insights?
> Thanks
> 
> On Mon, 27 May 2019 at 04:27, King Jiefei <szwjf08 using gmail.com> wrote:
> 
>> Hi Alexandre,
>>
>> I'm not an R expert so this is only my personal thought:
>>
>> I don't think you can achieve what you want exactly. A possible solution
>> would be defining a binary operator %*%, where you can replace the asterisk
>> with any function name you want. The function %*% is special since it has
>> two arguments, left operand and right operand respectively. You then
>> can call the `substitute` function to get its function arguments in an
>> expression format and proceed to do what you want. Here is an example to
>> show the idea.
>>
>> *Code:*
>>
>> `%myOperator%` <- function(x, y) {
>>    x = substitute(x)
>>    y = substitute(y)
>>    return(list(x, y))
>> }
>>
>>
>> myIf(i == 1, arg1) %myOperator% {
>>    doSomeThing
>> }
>>
>>
>> *Results:*
>>
>> [[1]]
>> myIf(i == 1, arg1)
>>
>> [[2]]
>> {
>>      doSomeThing
>> }
>>
>> I hope that helps.
>>
>> Best,
>> Jiefei
>>
>> On Sun, May 26, 2019 at 4:45 AM Alexandre Courtiol <
>> alexandre.courtiol using gmail.com> wrote:
>>
>>> Hi all,
>>>
>>> Could anyone refer to me to a good source to learn how to program a simple
>>> control-flow construct* in R, or provide me with a simple example?
>>>
>>> Control-flow constructs are programmed as primitives, but I would like to
>>> be able to do that (if possible) in pure R.
>>>
>>> The general context is that those functions are a mystery to me. The
>>> motivating example is that I would like to create a function that behave
>>> similarly to base::`if` with an extra argument to the function (e.g. to
>>> include an error rate on the condition).
>>>
>>> Many thanks,
>>>
>>> Alex
>>>
>>> * control-flow constructs are functions such as if, for, while... that
>>> allow for call of the form fn(x) expr to work (see ?Control).
>>>
>>> --
>>> Alexandre Courtiol
>>>
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>>
>