Problems With Higher Order Functions in tailr

Ok, there is a problem with higher-order functions in my tailr package that I ran into while writing linked list functions for my matchbox package.¹

If you write a tail-recursive function that uses a parameter-function, the tailr package will complain that it doesn’t see that function in scope. This is correct, but only because I do not check the formal arguments of the recursive function.

Consider this example, that involves a perfectly valid tail-recursive function for mapping over a linked list:

library(pmatch)

 llist := NIL | CONS(car, cdr:llist)

llrev <- function(llist, acc = NIL) {
    pmatch::cases(
        llist,
        NIL -> acc,
        CONS(car, cdr) -> llrev(cdr, CONS(car, acc))
    )
}
llrev <- tailr::loop_transform(llrev)

llmap <- function(llist, f, acc = NIL) {
    pmatch::cases(
        llist,
        NIL -> llrev(acc),
        CONS(car, cdr) -> llmap(cdr, f, CONS(f(car), acc))
    )
}

Trying to transform llmap will give you an error because tailr can’t find f.

I’ve put that on the todo list and it should be easy enough to fix. I just have to pass the function along to all the function calls so I can get at the formals(). I tried to fix it quickly, but changing the functions break everything, so I will have to do it when I have more time for it.