Re: [Haskell-cafe] Type systems preventing laziness-related memory leaks?

20 Feb 2015

      On 20/02/15 07:26, Eugene Kirpichov wrote:
...
On Tue Feb 17 2015 at 11:50:20 PM Roman Cheplyaka mailto:roma@ro-che.info> wrote:
Note that foldr (+) 0 has nothing to do with laziness — it's eager. Its
    problem is that it's not tail-recursive.
The problem is that when you say foldr (+) 0 [1, 2, 3, 4, 5] you build a
thunk "1 + (2 + (3 + (4 + (5 + 0))))", which has - let's call it "whnf
evaluation depth" of 4 (you need a stack of size 4 to evaluate it to whnf).
This is not a thunk. These values would be pushed to the stack, but no
thunks will be created.

Just so that you have no doubts left, here's the STG code for foldr (+) 0:

$wgo = \r srt:SRT:[] [w]
  case w of _ {
    [] -> 0;
    : y ys ->
      case y of _ {
        I# x -> case $wgo ys of ww {
          __DEFAULT -> +# [x ww];
        };
      };
  };

As you can see, there are no lets here, only cases.

Roman