It would be great to have a video of this in action up on youtube. You can simply 'recordmydesktop' on linux (and likely elsewhere), then upload the result. It also helps the general adoption cause, having Haskell more visible and accessible. claus.reinke:
The problem occurs when the result value is needed and thus the thunks need to be reduced, starting with the outermost, which can't be reduced without reducing the next one .... etc and it's these reduction steps that are pushed on the stack until its size cause a stack-overflow.
Yes, that's exactly right, and something that's not often pointed out.
Btw, this is kind of relative strictness (when is one part of my program needed to answer demands on another part) is the kind of example for which old GHood can be helpful (once you get used to the display).
If you have Java on your machines, try installing GHood [1] (on hackage thanks to Hugo Pacheco), then things like
ghc -e ':m +Debug.Observe' -e 'printO $ observe "foldr" foldr (+) 0 [1..4] ' ghc -e ':m +Debug.Observe' -e "printO $ observe \"foldl'\" foldl' (+) 0 [1..4] " ghc -e ':m +Debug.Observe' -e 'printO $ observe "foldl" foldl (+) 0 [1..4] '
This was also among the examples on the GHood home page [2], so you could try the applet version instead, and in section 4.2 of the paper [3] (as a "well known strictness problem";-). Page and paper mention a few other similar examples and discuss some differences between static (which parts are needed at all) and dynamic strictness (which parts are needed when, relative to other demands).
Claus
[1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GHood [2] http://www.cs.kent.ac.uk/~cr3/toolbox/haskell/GHood [3] http://www.cs.kent.ac.uk/~cr3/publications/GHood.html
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