[Haskell-cafe] space leak with 'concat' ?

[Jonathan Cast]

On Tue, 2009-01-27 at 22:12 +0100, Henning Thielemann wrote:
> $ ghc +RTS -M16m -c30 -RTS -e 'concat $ repeat "bla"'
> 
> This breaks down after a while, also if I increase the memory restriction:
> 
> ...
> ablablablablablablablablablablablablablablablablablablablablablaHeap exhausted;
> Current maximum heap size is 15998976 bytes (15 Mb);
> use `+RTS -M<size>' to increase it.
> 
> 
> Whereas this one works:
> 
> $ ghc +RTS -M16m -c30 -RTS -e 'cycle "bla"'

> 'concat' seems to be the culprit. What's so bad about it?

cycle builds a circular list:

  cycle xn = let ys = xn ++ ys in ys

which concat isn't smart enough to do (obviously).  That's not an issue
normally, since the list is being consumed in a properly lazy fashion.
But it looks like, for some reason, ghc -e is holding a pointer to the
entire expression-to-evaluate in this case, so cons cells that have
already been printed don't get garbage collected.  To show that there's
nothing wrong with concat per se, try this version instead:

  ghc +RTS -M16m -c30 -RTS -e 'print $ concat $ repeat "bla"'

This should print forever without any problems.

jcc