Friendlier compose()

[egnha]

It would be nice to have a "friendlier" version of compose() that:

• sets the formals to that of the first called function
• is more consistent with tidyverse semantics (i.e., supports splicing and formula syntax)
• has a more informative print method
• works as a drop-in replacement

To satisfy these desiderata (the second of which I presume is planned), I have something like the following in mind, which is a minor tweaking of the current implementation:

library(rlang)

compose <- function(...) {
  fs <- lapply(dots_list(...), as_function)
  n <- length(fs)
  
  last <- as_closure(fs[[n]])
  `__call_last` <- function() {
    call_last <- mut_node_car(sys.call(-1), last)
    eval_bare(call_last, parent.frame(2))
  }
  `__rest` <- rev(fs[-n])
  
  set_attrs(
    `formals<-`(
      value = formals(last),
      function() {
        out <- `__call_last`()
        for (f in `__rest`)
          out <- f(out)
        out
      }
    ),
    class = "composite_function"
  )
}

decompose <- function(x) {
  if (!inherits(x, "composite_function"))
    abort("Not a composite function")
    
  environment(x)$fs
}

print.composite_function <- function(x, ...) {
  cat("<composite_function>\n")
  cat("(Listed in calling order)\n")
  
  for (f in rev(decompose(x))) {
    cat("\n")
    print(f)
  }
  
  invisible(x)
}

(The method for formals setting is the same as in #349.)

Example:

foo <- compose(log, ~abs(.) + 1)

foo
#> <composite_function>
#> (Listed in calling order)
#> 
#> function (..., .x = ..1, .y = ..2, . = ..1) 
#> abs(.) + 1
#> 
#> function (x, base = exp(1))  .Primitive("log")

args(foo)
#> function (..., .x = ..1, .y = ..2, . = ..1) 
#> NULL

decompose(foo)
#> [[1]]
#> function (x, base = exp(1))  .Primitive("log")
#> 
#> [[2]]
#> function (..., .x = ..1, .y = ..2, . = ..1) 
#> abs(.x) + 1

[lionel-]

Invoking mut_node_car() in __call_last() is problematic, because a function in the composition could inspect the call stack. Instead, we should copy sys.call(-1):

yes if you're going to modify a call from the stack you should always rlang::duplicate() it.

[egnha]

I see, thanks for the tip!

If I've used duplicate() correctly, then I think the following should now be safe.

compose <- function(...) {
  fs <- lapply(dots_list(...), rlang::as_function)
  n <- length(fs)

  last <- as_closure(fs[[n]])
  `__call_last` <- function() {
    call <- duplicate(sys.call(-1))
    eval_bare(mut_node_car(call, last), parent.frame(2))
  }
  `__rest` <- rev(fs[-n])

  set_attrs(
    `formals<-`(
      value = formals(last),
      function() {
        out <- `__call_last`()
        for (f in `__rest`)
          out <- f(out)
        out
      }
    ),
    class = "composite_function"
  )
}

(Not 100% sure there isn't some "pathologically" impure function that could spoil this ... )

[egnha]

The proposed compose(), like purrr's current one, implicitly assumes referential transparency of the functions composed (since the function evaluations, aside from the the first, don't happen in the calling frame). Would be helpful for the doc to mention that (perhaps under “Details”?).

[lionel-]

That's a general assumptions of function operators, not sure what is the right place to document this. Also introspective and even reflexive functions might still work, there's just no guarantees.

By the way could you use more intermediary results in your code? They help the reader, especially with well chosen names. Also don't call formals<- with prefix form ;)

[egnha]

I was trying to keep the formals adjacent to the function declaration, but I do agree that it's unconventional. Consider it gone. ;)

Does this read better? Any further suggestions?

compose <- function(...) {
  fs <- lapply(dots_list(...), rlang::as_function)
  n <- length(fs)

  last <- as_closure(fs[[n]])
  `__call_last` <- function() {
    call <- duplicate(sys.call(-1))
    eval_bare(mut_node_car(call, last), parent.frame(2))
  }
  `__rest` <- rev(fs[-n])

  composite <- function() {
    out <- `__call_last`()
    for (f in `__rest`)
      out <- f(out)
    out
  }
  formals(composite) <- formals(last)
  class(composite) <- "composite_function"

  composite
}

[lionel-]

You could use the fn particle for functions as in rlang. Also I would write the call mutation like this:

mut_node_car(call, last)
eval_bare(call, parent.frame(2))

to make it clearer that some side effects are going on.

I wonder if there's a better way to forward the arguments. The capture of the function call + evaluation in the parent frame seems brittle to me.

[egnha]

Thanks for those suggestions. Makes sense. I'll incorporate them.

This pass-arguments-then-call-in-parent-frame pattern is not uncommon for these kinds of functional operators. It does feel to me, too, like a bit of a contortion for something so simple. If there's a more robust, or succinct, way to do it, I would certainly be interested in that.

I trust that you are right when you say that the current method is potentially brittle, but I don't quite understand how ... :/

[egnha]

By the way, another change—improvement?—to compose() that might be worth considering, as well, would be to allow compose() itself to be more composable, by avoiding the accumulation of nested calls (composition being associative).

Say, something like this: instead of

fs <- lapply(dots_list(...), rlang::as_function)

one could try unpacking any nested compositions

fs <- flatten_fns(...)

where

flatten_fns <- function(...) {
  fns <- lapply(dots_list(...), as_fn_decomposed)
  flatten(fns)
}
as_fn_decomposed <- function(x) {
  if (inherits(x, "composite_function"))
    decompose(x)
  else
    rlang::as_function(x)
}

This won't unpack compositions beyond the first level, however. (Would need to decompose recursively.)

[lionel-]

I like the forwarding of formals but I think there's not much gain from a better print method here. If we really needed it it would probably be better to use expr_interp() on the returned function so that its source code appears like this:

compose(log, ~abs(.) + 1)
compose(log, ~abs(.) + 1)
#> function(x, base = exp(1))
#> {
#>     x <- .Primitive("log")(x = x, base = base)
#>     x <- (function (..., .x = ..1, .y = ..2, . = ..1)
#>       abs(.) + 1)(x)
#>
#>     x
#> }

A better source could be produced by specifying names to functions and storing them in the enclosure of the composite function.

[lionel-]

One more thought, there is no need to duplicate the whole call so you can use shallow = TRUE. Or this would be my preference: new_language(last, node_cdr(call)).

You only need to duplicate the parents of the node you're mutating, and here call is the root of the call tree.

[egnha]

I wasn't aware of new_language(), thanks for pointing that out.

Incorporating your recommendations, compose() now looks like this:

compose <- function(...) {
  fns <- flatten_fn_list(...)
  n <- length(fns)

  fn_last <- as_closure(fns[[n]])
  `__call_fn_last` <- function() {
    call <- new_language(fn_last, node_cdr(sys.call(-1)))
    eval_bare(call, parent.frame(2))
  }
  `__fns_rest` <- rev(fns[-n])

  fn_comp <- function() {
    out <- `__call_fn_last`()
    for (f in `__fns_rest`)
      out <- f(out)
    out
  }
  formals(fn_comp) <- formals(fn_last)
  class(fn_comp) <- "composite_function"

  fn_comp
}

with the small addition of flattening of nested compositions (same as above, hopefully with better names):

flatten_fn_list <- function(...) {
  fns <- lapply(dots_list(...), as_decomposed_function)
  unlist(fns)
}

as_decomposed_function <- function(x) {
  if (inherits(x, "composite_function"))
    decompose(x)
  else
    rlang::as_function(x)
}

(Contrary to my previous claim, the flattening happens all the way down.)

[egnha]

With flattening, the following all yield the same function.

compose(log, abs, sin, `+`)
compose(log, abs, sin, compose(`+`))
compose(log, abs, compose(sin,  compose(`+`)))
compose(log, compose(abs, compose(sin,  compose(`+`))))

In particular, the call modifier `__call_fn_last`() is only invoked once.

[egnha]

Your suggested print method is an interesting idea; it certainly more accurately reflects the underlying action than just listing the functions.

Another thought: If there are a lot of functions, or if their bodies are long, some kind of output truncation could be useful.

In any case, I think some kind of specialized print method is helpful for compositions, because otherwise you'd just see this:

print.default(compose(log, abs, sin, `+`))
#> function (.x, .y) 
#> {
#>   out <- `__call_fn_last`()
#>   for (f in `__fns_rest`) out <- f(out)
#>   out
#> }
#> <environment: 0x7fccae013e98>
#>   attr(,"class")
#> [1] "composite_function"

[lionel-]

I think we should inline the function literal so you would see the source. And optionally a symbol if the function was supplied with a name. This way we don't need a print method and s3 class.