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

Thomas Mailund
Wed, 10 Aug 2016 17:10:43 -0000

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 <murdoch.duncan at
gmail.com> wrote:
>
> On 10/08/2016 12:53 PM, Thomas Mailund 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.
>
> 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
>
>