Continuation-parsing / trampoline / infinite recursion problem

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.