In Haskell the foldl' function defined in the module Data.List is better than foldl because that does not use a thunk. A thunked expression requires an internal stack. As an expression can grow infinitely large, the runtime imposes a limit on the size of this stack. As the simple example below shows that given a large enough input the stack will overflow.
Prelude> foldr (+) 0 [1..100]
5050
Prelude> foldl (+) 0 [1..100]
5050
Prelude> foldl (+) 0 [1..1000]
500500
Prelude> foldl (+) 0 [1..10000]
50005000
Prelude> foldl (+) 0 [1..100000]
5000050000
Prelude> foldl (+) 0 [1..1000000]
*** Exception: stack overflow
On the other hand, foldl' while similar to foldl does not build up on thunks and in real world programs is probably more useful.
Prelude> :module +Data.List
Prelude Data.List> foldl' (+) 0 [1..1000000]
500000500000
Prelude Data.List> foldl' (+) 0 [1..1000000]
500000500000
Prelude Data.List> foldl' (+) 0 [1..10000000]
50000005000000
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