On 03/11/14 18:16, Henning Thielemann wrote:
If 'seq' is faster than 'deepseq' it leaves things unevaluated and thus builds up unevaluated expressions, right?
Yes. What I mean is you could write a function function (using repeated foldl' inside) that is written such that its return value is fully evaluated (by carefully using just as many seqs as necessary for that). If foldl' did a full deepseq on its return value, then this function would seq the same data more times than necessary. I will try to give a (probably not so excellent) example: Let's say you want to `fmap (*2)` over a tree and implement it in recursive `foldl' (:)` style over the children of each node. Then deepseq in foldl' would trigger a full traversal of every subtree in the tree, adding a (*n) runtime factor. Yes, in this case you could probably just use laziness all well, using only the current foldl' and do deepseq outside, but I can imagine that there are cases similar to my example where that doesn't work, and were you want explicit seq control (but I can't come up with a perfect example for this yet).