How to getPar and setPar to access the guts of par()

Question

I am working on some generic accessor functions for the par() option in R.

getPar

getPar = function(key)
    {
    par()[[key]];   
    }

This works as expected.

getPar("mar");

setPar

# save memory ... restoreState ... pdf 
setPar = function(key, val)
    {
    # par(mar=c(0.25, 0.25, 0.25, 0.25)
    # R.O. indicates read-only arguments: These may only be used in queries and cannot be set. ("cin", "cra", "csi", "cxy", "din" and "page" are always read-only.)
    # https://www.rdocumentation.org/packages/graphics/versions/3.6.2/topics/par
    pnames = names( par(no.readonly = TRUE) );
    if(is.element(key, pnames))
        {
        par()[[key]] = val;
        }
    }

This one does not work:

mar = c(0.25, 0.25, 0.25, 0.25);
setPar("mar", mar);

And throws an error:

Error in par()[[key]] = val : invalid (NULL) left side of assignment

Any ideas on how I can write a setter function as outlined?

Yes, I understand, I can pass par(mar = c(0.25, 0.25, 0.25, 0.25)) directly to "set" the value. I am specifically looking for a solution that will work inside this simple setter function. A variadic approach.

Answer 1

The assignment to lists (i.e., [<- and [[<-) requires an object on the LHS of the assignment, not an expression that produces the list. Similarly,

A <- list(a=1)
A$b <- 2
A
# $a
# [1] 1
# $b
# [1] 2
list(a=1)$b <- 3
# Error in list(a = 1)$b <- 3 : 
#   target of assignment expands to non-language object

I suggest you change setPar to actually set the value using par, not try to operate on the list.

setPar = function(key, val)
    {
    # par(mar=c(0.25, 0.25, 0.25, 0.25)
    # R.O. indicates read-only arguments: These may only be used in queries and cannot be set. ("cin", "cra", "csi", "cxy", "din" and "page" are always read-only.)
    # https://www.rdocumentation.org/packages/graphics/versions/3.6.2/topics/par
    pnames = names( par(no.readonly = TRUE) );
    if(is.element(key, pnames))
        {
        par(setNames(list(val), key))
        }
    }

par("mar")
# [1] 5.1 4.1 4.1 2.1
setPar("mar", 1:4)
par("mar")
# [1] 1 2 3 4