On 26-7-2013 11:54, [email protected] wrote:
OK, the problem is not that 1000000 dropped elements would be lingering in the
heap.
The problem is that element 1000001 is not actually evaluated until it is
really realy needed by
Start = hd ..
Until then, element 1000001 is a huge thunk 1+1+1+1... (consisting of 1000001
numbers and 1000000 references to the +-operator).
Yes.
Say we would have a version of gen_list that would be strict in its argument.
Could we write a program that would not terminate, while a version with a lazy
gen_list would?
Yes, for example:
undef_int :: Int
undef_int = undef
Start = isEmpty [undef_int..]
To prevent the large 1+1+1+1.. lazy computation, you could use:
gen_list n = gen_int_list 0 n
where
gen_int_list :: !Int Int -> [Int]
gen_int_list a n
= [n+a : gen_int_list (a+1) n]
Kind regards,
John van Groningen
-----Oorspronkelijk bericht-----
Van: [email protected] namens [email protected]
Verzonden: ma 22-7-2013 17:11
Aan: Clean List
Onderwerp: Re: [clean-list] hd (drop 1000000 [1..]) heapFull
So is this desired behaviour, in the context of hd?
________________________________
On 19-7-2013 22:10, Pieter Koopman wrote:
Hi Erik,
there seems to be a problem with the generator.
Start = hd (drop n [1..n+10]) where n = 1000000
works fine. Hopefully John can explain this.
[1..] generates a list in the following way:
let
gen_list n = [n : gen_list (n+1)]
in
gen_list 1
Because gen_list is not strict in argument n,
a thunk is created for the expression (n+1) at runtime.
The first time this creates a thunk t0 with 1+1,
the next time a thunk t1 with t0+1, then t2 with
t1+1, ..
So after 1000000 elements, the heap contains 1000000
thunks.
The arguments of the function generated for [1..n+10] are strict,
because 1 is compared with n+10, so no increment thunks
are created.
On 7/19/13 4:41 PM, [email protected] wrote:
Re: [clean-list] Matrix operations
Start = hd (drop 1000000 [1..])
with standard heap (2M) leads to a heapfull message; not when I only
drop 1000.
I had expected the garbage collector to kick in so this would
effectively run in constant space
Any ideas?
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