On Apr 15, 2005, at 7:03 PM, Jan-Willem Maessen wrote:
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.
I was writing in the context of the yield with Monad.Cont example. The 'it' in 'whether it run in constant space' is the yield example of "length $ take (10^7) zerosInf". Nothing more general. So I agree with your statement, that asymptotic space consumption can change with optimization. The odd thing is that ghc (without -O) compiles and run the example in question, but ghci will fail. So I was wondering if there was an implementation of yield using continuations that would make yield space efficient in ghci. Perhaps I should also check hugs, but I have not used hugs yet. -- Chris