On Apr 15, 2005, at 6:07 PM, ChrisK wrote:
You are correct. Moand.Cont yield even runs without -O optimizing, just slower ... Anyone have an idea why ghci can't garbage collect it? Is this an actual bug or an innate quirk of the REPL ?
GHCi does not "compile" with optimizations, without -O the strictness analyzer
isn't run.
The optimizer is irrelevant to whether it runs in constant space, as ghc without '-O' runs it just fine. The optimizer is only useful for speed, which is not the issue.
Not true! The optimizer can change the asymptotic space consumption of your program. The example of sum is particularly germaine. If we write:
x = foldl (+) 0 [1..n]
this will, as it is evaluated, generate the suspended computation (((....((((0 + 1) + 2) + 3) + ...) + n
This require O(n) space.
Whereas the strict foldl' will evaluate each parenthesized expression as it is encountered:
(0+1) = 1
(1+2) = 3
(3+3) = 6
(6+4) = 10
...
(... + n) = your answer
If only we all used mostly-eager evaluation, these kinds of confusions would [almost] never happen.
-Jan-Willem Maessen
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