On 10/08/2016 2:39 PM, Thomas Mailund wrote:
Ok, I think maybe I am beginning to see what is going wrong...
Explicitly remembering the thunk parameters in a list works fine, as far as I
can see.
make_thunk <- function(f, ...) {
remembered <- list(...)
function(...) do.call(f, as.list(remembered))
}
Where that will fail is in a situation like this:
thunklist <- list(thunk_factorial, thunk_somethingelse)
for (i in seq_along(thunklist))
thunklist[[i]] <- make_thunk(thunklist[[i]])
The problem is that the first time thunklist[[1]] is evaluated, it will
call the function thunklist[[2]] (or something else if i has been
modified in the meantime), and things will go bad. That's why it's
important to force both f and ... in make_thunk.
Duncan Murdoch
thunk_factorial <- function(n, continuation = identity) {
if (n == 1) {
continuation(1)
} else {
new_continuation <- function(result) {
make_thunk(continuation, n * result)
}
make_thunk(thunk_factorial, n - 1, new_continuation)
}
}
trampoline <- function(thunk) {
while (is.function(thunk)) thunk <- thunk()
thunk
}
trampoline(thunk_factorial(100))
But if I delay the evaluation of the parameters to thunk I get an error
make_thunk <- function(f, ...) {
remembered <- eval(substitute(alist(...))) # not evaluating parameters yet
function(...) do.call(f, as.list(remembered))
}
thunk_factorial <- function(n, continuation = identity) {
if (n == 1) {
continuation(1)
} else {
new_continuation <- function(result) {
make_thunk(continuation, n * result)
}
make_thunk(thunk_factorial, n - 1, new_continuation)
}
}
trampoline(thunk_factorial(100))
Running this version I am told, when applying the function, that it doesn’t see
variable `n`.
As far as I can see, the thunk remembers the parameters just fine. At least
this gives me the parameters I made it remember
x <- 1
f <- make_thunk(list, a = 1 * x, b = 2 * x)
g <- make_thunk(list, c = 3 * x)
f()
g()
Here I just get the parameters back in a list because the wrapped function is
`list`. (The reason I have `x` as a global variable and use it in the arguments
is so I get call objects that needs to be evaluated lazily instead of just
values).
These values contain the expressions I gave the `make_thunk` function, of
course, and they are not evaluated. So in the factorial function the missing
`n` is because I give it the expression `n - 1` that it of course cannot
evaluate in the thunk.
So I cannot really delay evaluation.
Does this sound roughly correct?
Now why I can still get it to work when I call `cat` remains a mystery…
Cheers
Thomas
On 10 August 2016 at 19:12:41, Thomas Mailund
(mail...@birc.au.dk(mailto:mail...@birc.au.dk)) 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.
Thanks a lot!
Thomas
On 10 Aug 2016, at 19:04, Duncan Murdoch wrote:
On 10/08/2016 12:53 PM, Thomas Mailund wrote:
On 10 Aug 2016, at 13:56, Thomas Mailund 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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