[R] Continuation-parsing / trampoline / infinite recursion problem

Thomas Mailund mailund at birc.au.dk
Wed Aug 10 20:39:03 CEST 2016

```
Ok, I think maybe I am beginning to see what is going wrong...

Explicitly remembering the thunk parameters in a list works fine, as far as I can see.

make_thunk <- function(f, ...) {
remembered <- list(...)
function(...) do.call(f, as.list(remembered))
}

thunk_factorial <- function(n, continuation = identity) {
if (n == 1) {
continuation(1)
} else {
new_continuation <- function(result) {
make_thunk(continuation, n * result)
}
make_thunk(thunk_factorial, n - 1, new_continuation)
}
}

trampoline <- function(thunk) {
while (is.function(thunk)) thunk <- thunk()
thunk
}

trampoline(thunk_factorial(100))

But if I delay the evaluation of the parameters to thunk I get an error

make_thunk <- function(f, ...) {
remembered <- eval(substitute(alist(...))) # not evaluating parameters yet
function(...) do.call(f, as.list(remembered))
}

thunk_factorial <- function(n, continuation = identity) {
if (n == 1) {
continuation(1)
} else {
new_continuation <- function(result) {
make_thunk(continuation, n * result)
}
make_thunk(thunk_factorial, n - 1, new_continuation)
}
}

trampoline(thunk_factorial(100))

Running this version I am told, when applying the function, that it doesn’t see variable `n`.

As far as I can see, the thunk remembers the parameters just fine. At least this gives me the parameters I made it remember

x <- 1
f <- make_thunk(list, a = 1 * x, b = 2 * x)
g <- make_thunk(list, c = 3 * x)
f()
g()

Here I just get the parameters back in a list because the wrapped function is `list`. (The reason I have `x` as a global variable and use it in the arguments is so I get call objects that needs to be evaluated lazily instead of just values).

These values contain the expressions I gave the `make_thunk` function, of course, and they are not evaluated. So in the factorial function the missing `n` is because I give it the expression `n - 1` that it of course cannot evaluate in the thunk.

So I cannot really delay evaluation.

Does this sound roughly correct?

Now why I can still get it to work when I call `cat` remains a mystery…

Cheers
Thomas

On 10 August 2016 at 19:12:41, Thomas Mailund (mailund at birc.au.dk(mailto:mailund at birc.au.dk)) wrote:

>
> That did the trick!
>
> I was so focused on not evaluating the continuation that I completely forgot that the thunk could hold an unevaluated value… now it seems to be working for all the various implementations I have been playing around with.
>
> I think I still need to wrap my head around *why* the forced evaluation is necessary there, but I will figure that out when my tired brain has had a little rest.
>
> Thanks a lot!
>
> Thomas
>
>
> > On 10 Aug 2016, at 19:04, Duncan Murdoch wrote:
> >
> > On 10/08/2016 12:53 PM, Thomas Mailund wrote:
> >> > On 10 Aug 2016, at 13:56, Thomas Mailund wrote:
> >> >
> >> > make_thunk <- function(f, ...) f(...)
> >>
> >> Doh! It is of course this one:
> >>
> >> make_thunk <- function(f, ...) function() f(…)
> >>
> >> It just binds a function call into a thunk so I can delay its evaluation.
> >
> > I haven't looked closely at the full set of functions, but this comment:
> >
> > force(continuation) # if I remove this line I get an error
> >
> > makes it sound as though you're being caught by lazy evaluation. The "make_thunk" doesn't appear to evaluate ..., so its value can change between the time you make the thunk and the time you evaluate it. I think you could force the evaluation within make_thunk by changing it to
> >
> > make_thunk <- function(f, ...) { list(...); function() f(…) }
> >
> > and then would be able to skip the force() in your thunk_factorial function.
> >
> > Duncan Murdoch
> >
> >
>
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