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

Thomas Mailund
Wed, 10 Aug 2016 16:59:20 -0000

An alternative implementation, closer to what I need when I have more than one
recursion in each step, but still using factorial as the example, is this one:

thunk_factorial <- function(n, continuation = identity) {
  force(continuation) # if I remove this line I get an error
  cat("call: ", n, "\n") # same for this line
  if (n == 1) {
    continuation(1)
  } else {
    new_continuation <- function(result) {
      cat("thunk: ", result, "\n?) # remove this line and it
fails, keep it and it works
      make_thunk(continuation, n * result)
    }
    make_thunk(thunk_factorial, n - 1, new_continuation)
  }
}
trampoline(thunk_factorial(10000))

Here I am making a continuation instead of passing along an accumulator, which I
need to do for more complex cases, and with that continuation I can also get it
to complete without errors if I output the text inside it. Removing the `cat`
line and I get the recursion error?

Cheers
	Thomas



> On 10 Aug 2016, at 18:53, Thomas Mailund <mailund at birc.au.dk>
wrote:
>
>
>> On 10 Aug 2016, at 13:56, Thomas Mailund <mailund at birc.au.dk>
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.
>
> Sorry
> 	Thomas
>
>
>
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