[Haskell-cafe] missing rseq?

Yuras Shumovich shumovichy at gmail.com
Wed Mar 11 14:11:51 UTC 2015

On Wed, 2015-03-11 at 17:07 +0300, Yuras Shumovich wrote:
> On Wed, 2015-03-11 at 08:26 +0100, Kees Bleijenberg wrote:
> > In the book Parallel and Concurrent  Programming in Haskell
> > http://chimera.labs.oreilly.com/books/1230000000929/ch02.html#sec_par-eval-s
> > udoku2 a list of sudokus is solved in parallel.
> > 
> > In version 2 (sudoku2.hs) the program splits the list of sudokus in 2
> > seperate lists and solves these lists in parallel.
> > 
> > In version3 (sudoku3.hs) parMap is used. 
> > 
> > What I don't understand is why in sudoku2 the program has to wait until the
> > parallel computations are finished with rseqs while in sudoku3.hs there is
> > no rseq (not in the main program nor in parMap)? Why can't program
> > sudoku3.hs terminate before all parallel calculations are finished as in
> > Example 2-1. rpar/rpar?
> I believe `rseq` is omitted in `sudoku3` for a reason.
> In `sudoku2` we spawn two relatively heavy sparks, and the results of
> them are immediately consumed. So if we don't wait for the sparks, the
> main thread will start evaluating one of the thunks. Most likely the
> thunk will be evaluated twice -- by the main thread and the spark. That
> is called `fizzled` spark.
> In `sudoku3` we have a lot of light sparks. When we spawn the last
> spark, the first one most like is already evaluated (aka `converted`),
> and its result can be immediately consumed by `length`.

Also I should note that `rseq` seems to be broken now, because it always
produces one `converted` and one `fizzled` spark for `sudoku2` in my

> Thanks,
> Yuras

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