The semantics of foldl' for lists were changed between base 4.7 and base 4.8. Specifically, foldl' became strict in the initial value of its accumulator. I opened http://ghc.haskell.org/trac/ghc/ticket/12173 to report this. The change was entirely accidental, according to Joachim Breitner. However, Duncan Coutts indicated he is pleased with the change. I don't personally have a dog in this race, but I feel very strongly about three things: 1. The strictness should be fully documented, both in Haddock and the next Haskell Report (the Haskell 2010 Report does not go into sufficient detail to support either choice). 2. There should be *one* meaning of foldl' in base. Thus the default Foldable instance should match the ones for lists and arrays. 3. The containers package should be consistent with base in this regard.