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
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