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.
Duncan Murdoch
Thanks a lot!
Thomas
> On 10 Aug 2016, at 19:04, Duncan Murdoch <murdoch.dun...@gmail.com> wrote:
>
> On 10/08/2016 12:53 PM, Thomas Mailund wrote:
>> > On 10 Aug 2016, at 13:56, Thomas Mailund <mail...@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
>
>
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