Evaluating lazily 'f<-' ?

Hello Andrew,


I try now to understand the evaluation of the expression:

e = expression(r(x) <- 1)

# parameter named "value" seems to be required;
'r<-' = function(x, value) {print("R");}
eval(e, list(x=2))
# [1] "R"

# both versions work
'r<-' = function(value, x) {print("R");}
eval(e, list(x=2))
# [1] "R"


### the Expression
e[[1]][[1]] # "<-", not "r<-"
e[[1]][[2]] # "r(x)"


The evaluation of "e" somehow calls "r<-", but evaluates also the
argument of r(...). I am still investigating what is actually happening.


Sincerely,


Leonard


On 9/13/2021 9:15 PM, Andrew Simmons wrote:

R's parser doesn't work the way you're expecting it to. When doing an
assignment like:


padding(right(df)) <- 1


it is broken into small stages. The guide "R Language Definition"
claims that the above would be equivalent to:


`<-`(df, `padding<-`(df, value = `right<-`(padding(df), value = 1)))


but that is not correct, and you can tell by using `substitute` as you
were above. There isn't a way to do what you want with the syntax you
provided, you'll have to do something different. You could add a
`which` argument to each style function, and maybe put the code for
`match.arg` in a separate function:


match.which <- function (which)
match.arg(which, c("bottom", "left", "top", "right"), several.ok = TRUE)


padding <- function (x, which)
{
    which <- match.which(which)
    # more code
}


border <- function (x, which)
{
    which <- match.which(which)
    # more code
}


some_other_style <- function (x, which)
{
    which <- match.which(which)
    # more code
}


I hope this helps.

On Mon, Sep 13, 2021 at 12:17 PM Leonard Mada <leo.mada at syonic.eu
<mailto:leo.mada at syonic.eu>> wrote:

    Hello Andrew,


    this could work. I will think about it.


    But I was thinking more generically. Suppose we have a series of
    functions:
    padding(), border(), some_other_style();
    Each of these functions has the parameter "right" (or the group of
    parameters c("right", ...)).


    Then I could design a function right(FUN) that assigns the value
    to this parameter and evaluates the function FUN().


    There are a few ways to do this:

    1.) Other parameters as ...
    right(FUN, value, ...) = value; and then pass "..." to FUN.
    right(value, FUN, ...) = value; # or is this the syntax? (TODO:
    explore)

    2.) Another way:
    right(FUN(...other parameters already specified...)) = value;
    I wanted to explore this 2nd option: but avoid evaluating FUN,
    unless the parameter "right" is injected into the call.

    3.) Option 3:
    The option you mentioned.


    Independent of the method: there are still weird/unexplained
    behaviours when I try the initial code (see the latest mail with
    the improved code).


    Sincerely,


    Leonard


    On 9/13/2021 6:45 PM, Andrew Simmons wrote:

    I think you're trying to do something like:

    `padding<-` <- function (x, which, value)
    {
        which <- match.arg(which, c("bottom", "left", "top",
    "right"), several.ok = TRUE)
        # code to pad to each side here
    }

    Then you could use it like

    df <- data.frame(x=1:5, y = sample(1:5, 5))
    padding(df, "right") <- 1

    Does that work as expected for you?

    On Mon, Sep 13, 2021, 11:28 Leonard Mada via R-help
    <r-help at r-project.org <mailto:r-help at r-project.org>> wrote:

        I try to clarify the code:


        ###
        right = function(x, val) {print("Right");};
        padding = function(x, right, left, top, bottom)
        {print("Padding");};
        'padding<-' = function(x, ...) {print("Padding = ");};
        df = data.frame(x=1:5, y = sample(1:5, 5)); # anything

        ### Does NOT work as expected
        'right<-' = function(x, value) {
             print("This line should be the first printed!")
             print("But ERROR: x was already evaluated, which printed
        \"Padding\"");
             x = substitute(x); # x was already evaluated before
        substitute();
             return("Nothing"); # do not now what the behaviour
        should be?
        }

        right(padding(df)) = 1;

        ### Output:

        [1] "Padding"
        [1] "This line should be the first printed!"
        [1] "But ERROR: x was already evaluated, which printed
        \"Padding\""
        [1] "Padding = " # How did this happen ???


        ### Problems:

        1.) substitute(x): did not capture the expression;
        - the first parameter of 'right<-' was already evaluated,
        which is not
        the case with '%f%';
        Can I avoid evaluating this parameter?
        How can I avoid to evaluate it and capture the expression:
        "right(...)"?


        2.) Unexpected
        'padding<-' was also called!
        I did not know this. Is it feature or bug?
        R 4.0.4


        Sincerely,


        Leonard


        On 9/13/2021 4:45 PM, Duncan Murdoch wrote:

        > On 13/09/2021 9:38 a.m., Leonard Mada wrote:

        >> Hello,
        >>
        >>
        >> I can include code for "padding<-"as well, but the error

        is before that,

        >> namely in 'right<-':
        >>
        >> right = function(x, val) {print("Right");};
        >> # more options:
        >> padding = function(x, right, left, top, bottom)

        {print("Padding");};

        >> 'padding<-' = function(x, ...) {print("Padding = ");};
        >> df = data.frame(x=1:5, y = sample(1:5, 5));
        >>
        >>
        >> ### Does NOT work
        >> 'right<-' = function(x, val) {
        >>         print("Already evaluated and also does not use

        'val'");

        >>         x = substitute(x); # x was evaluated before
        >> }
        >>
        >> right(padding(df)) = 1;

        >
        > It "works" (i.e. doesn't generate an error) for me, when I

        correct

        > your typo:  the second argument to `right<-` should be

        `value`, not

        > `val`.
        >
        > I'm still not clear whether it does what you want with that

        fix,

        > because I don't really understand what you want.
        >
        > Duncan Murdoch
        >

        >>
        >>
        >> I want to capture the assignment event inside "right<-"

        and then call

        >> the function padding() properly.
        >>
        >> I haven't thought yet if I should use:
        >>
        >> padding(x, right, left, ... other parameters);
        >>
        >> or
        >>
        >> padding(x, parameter) <- value;
        >>
        >>
        >> It also depends if I can properly capture the unevaluated

        expression

        >> inside "right<-":
        >>
        >> 'right<-' = function(x, val) {
        >>
        >> # x is automatically evaluated when using 'f<-'!
        >>
        >> # but not when implementing as '%f%' = function(x, y);
        >>
        >> }
        >>
        >>
        >> Many thanks,
        >>
        >>
        >> Leonard
        >>
        >>
        >> On 9/13/2021 4:11 PM, Duncan Murdoch wrote:

        >>> On 12/09/2021 10:33 a.m., Leonard Mada via R-help wrote:

        >>>> How can I avoid evaluation?
        >>>>
        >>>> right = function(x, val) {print("Right");};
        >>>> padding = function(x) {print("Padding");};
        >>>> df = data.frame(x=1:5, y = sample(1:5, 5));
        >>>>
        >>>> ### OK
        >>>> '%=%' = function(x, val) {
        >>>>        x = substitute(x);
        >>>> }
        >>>> right(padding(df)) %=% 1; # but ugly
        >>>>
        >>>> ### Does NOT work
        >>>> 'right<-' = function(x, val) {
        >>>>        print("Already evaluated and also does not use

        'val'");

        >>>>        x = substitute(x); # is evaluated before
        >>>> }
        >>>>
        >>>> right(padding(df)) = 1

        >>>
        >>> That doesn't make sense.  You don't have a `padding<-`

        function, and

        >>> yet you are trying to call right<- to assign something to

        padding(df).

        >>>
        >>> I'm not sure about your real intention, but assignment

        functions by

        >>> their nature need to evaluate the thing they are

        assigning to, since

        >>> they are designed to modify objects, not create new ones.
        >>>
        >>> To create a new object, just use regular assignment.
        >>>
        >>> Duncan Murdoch

        >

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