[Haskell] stack overflow - nonobvious thunks?
Chad.Scherrer at pnl.gov
Wed Jul 27 13:12:45 EDT 2005
I'm trying to write a function to build a table of values from a list.
Here's my current attempt:
table :: (Ord a) => [a] -> [(a,Int)]
table xs = Map.assocs $! foldl' f Map.empty xs
f m x = Map.insertWith (+) x 1 m
The ($!) and the foldl' were my clumsy attempts to avoid the stack
overflows I keep getting, but it's not working right yet. If I set
unif :: [Int]
unif = randomRs (1,10) $ mkStdGen 1
then I should be able to use
f :: Int -> [(Int, Int)]
f n = table $ take n unif
I would think this should work using very little memory, since unif is
evaluated lazily, and the table is built eagerly. But I must be keeping
around more thunks than I think, since I get (in ghci)
*Main> f 1000000
*Main> f 10000000
[(1,*** Exception: stack overflow
So it works on big lists, but not huge ones. What am I missing? Is there
a way to do this other than just increasing the memory allocation?
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