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

Thomas Mailund mailund at birc.au.dk
Thu Aug 11 23:15:38 CEST 2016

I don’t know… that was the behaviour I had yesterday, but on the laptop where I was doing the experiments I updated R from 3.2 to 3.3 earlier today and now the original make_thunk

make_thunk <- function(f, …) function() f(…)

also works for me.

I don’t know what else I can say to that :)


On 11 August 2016 at 23:05:29, Duncan Murdoch (murdoch.duncan at gmail.com<mailto:murdoch.duncan at gmail.com>) wrote:

On 10/08/2016 1:28 PM, Duncan Murdoch wrote:
> On 10/08/2016 1:10 PM, Thomas Mailund 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.
> The original version
> make_thunk <- function(f, ...) function() f(…)
> says to construct a new function whose body evaluates the expression
> f(...). It never evaluates f nor ... , so they don't get evaluated
> until the first time you evaluate that new function.
> My version containing list(...) forces evaluation of ... . It would
> have been even better to use
> make_thunk <- function(f, ...) { list(f, ...); function() f(…) }
> because that forces evaluation of both arguments.
> I suspect you would have problems with
> make_thunk <- function(f, ...) function() do.call(f, list(...))
> for exactly the same reasons as the original; I'm surprised that you
> found it appears to work.

I have done some experimentation, and am unable to reproduce the
behaviour you described. Using do.call() doesn't affect things.

Duncan Murdoch

	[[alternative HTML version deleted]]

More information about the R-help mailing list