Question

Just out of curiosity, I made a simple script to check speed and memory efficiency of constructing a list in Haskell:

wasteMem :: Int -> [Int]
wasteMem 0 = [199]
wasteMem x = (12432483483467856487256348746328761:wasteMem (x-1))

main = do
    putStrLn("hello")
    putStrLn(show (wasteMem 10000000000000000000000000000000000))

The strange thing is, when I tried this, it didn’t run out of memory or stack space, it only prints [199], the same as running wasteMem 0. It doesn’t even print an error message… why? Entering this large number in ghci just prints the number, so I don’t think it’s a rounding or reading error.

Answer 1

Your program is using a number greater than maxBound :: Int32. This means it will behave differently on different platforms. For GHC x86_64 Int is 64 bits (32 bits otherwise, but the Haskell report only promises 29 bits). This means your absurdly large value (1×10^34) is represented as 4003012203950112768 for me and zero for you 32-bit folks:

GHCI> 10000000000000000000000000000000000 :: Int
4003012203950112768
GHCI> 10000000000000000000000000000000000 :: Data.Int.Int32
0

This could be made platform independent by either using a fixed-size type (ex: from Data.Word or Data.Int) or using Integer.

All that said, this is a poorly conceived test to begin with. Haskell is lazy, so the amount of memory consumed by wastedMem n for any value n is minimal – it’s just a thunk. Once you try to show this result it will grab elements off the list one at a time – first generating "[12432483483467856487256348746328761, and leaving the rest of the list as a thunk. The first value can be garbage collected before the second value is even considered (a constant-space program).