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Fix dot documentation #32

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Fix dot documentation #32

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9a57483
Fix dot documentation
treeowl Jun 4, 2018
1e27bea
Simplify and extend
treeowl Jun 5, 2018
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17 changes: 13 additions & 4 deletions Control/Parallel/Strategies.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -296,13 +296,22 @@ withStrategy = flip using
-- | Compose two strategies sequentially.
-- This is the analogue to function composition on strategies.
--
-- For any strategies @strat1@, @strat2@, and @strat3@,
-- > strat2 `dot` strat1 == strat2 . withStrategy strat1
--
-- 'dot' is associative:
--
-- > (strat1 `dot` strat2) `dot` strat3 == strat1 `dot` (strat2 `dot` strat3)
-- > strat1 `dot` strat1 = strat1
-- > strat1 `dot` r0 == strat1
--
-- > strat2 `dot` strat1 == strat2 . withStrategy strat1
-- 'r0', 'rseq', and 'rpar' are all one-sided identities of 'dot':
--
-- > strat `dot` r0 == strat
-- > strat `dot` rseq == strat
-- > strat `dot` rpar == strat
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Is that true?

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@treeowl treeowl Jun 5, 2018 •
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I believe so, yes. runEval m is a lifted value; it doesn't do anything until it's forced. Forcing runEval (r0 x) runs the usual return x IO computation, so that gives you x. Forcing runEval (rseq x) runs an IO computation that forces x to WHNF and returns the result (exactly the same as forcing x directly). Forcing runEval (rpar x) runs an IO computation that sparks a thread to evaluate x to WHNF, but immediately forces the result so the thread (almost certainly) fizzles.

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r0 `dot` rseq == r0
r0 `dot` rpar == r0

Would be counterintuitive to my expectations, of the first description of dot, namely: "Compose two strategies sequentially". Or more explicitely:

"Composing the 'do nothing'-strategy with the 'evaluate to WHNF'-strategy sequentially, creates the 'do nothing'-strategy"

Am I the only, who thinks this sounds weird?

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@treeowl treeowl Jun 9, 2018 •
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r0 `dot` rseq is not r0. But rseq `dot` r0 = rseq.

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Last comment corrected.

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dot is a composition operator in a semigroupoids sense: namely, it is associative. But it doesn't form a category, which makes intuition around it a little tricky. @Borgvall, by definition,

f `dot` g = \x -> f (runEval (g x))

Think about what runEval (r0 x), runEval (rpar x), and runEval (rseq x) do when forced. Ultimately the same thing (if you ignore sparks that fizzle). If you don't force them, they all do ... nothing.

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I care about sparking, this is the main reason to use this library!

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I'm still concerned about strat dot rpar == strat

We can construct contrived examples in which this doesn't apply, at least operationally (and operationally is after all the point of the library, so the equalities should be operational ones). e.g.

let strat n x = rseq (fib n) >> return x  in strat 99999 `dot` rpar

If strat is very slow then the spark doesn't fizzle immediately and we get some useful parallelism. Of course it's hard to reliably define a strategy that's very slow (the example above could easily be optimised), but the fact that it's possible at all implies that we can't assume the spark fizzles immediately.

--
-- Furthermore, since strategies should only force and sparks computations
-- (that is, they don't actually change any values),
--
-- > strat `dot` strat == strat
--
dot :: Strategy a -> Strategy a -> Strategy a
strat2 `dot` strat1 = strat2 . runEval . strat1
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