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

[Roman Cheplyaka]

On 20/02/15 07:26, Eugene Kirpichov wrote:
> 
> 
> On Tue Feb 17 2015 at 11:50:20 PM Roman Cheplyaka <roma at ro-che.info
> <mailto:roma at 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