Difference between revisions of "Talk:Parallelism vs. Concurrency"
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m (Type error corrected) |
(Using unsafeInterleaveIO where applicable) |
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− | * Now have a look at this <del> |
+ | * Now have a look at this <del>nearly-as-ugly</del> prototype definition for <del><code>spawnIO</code></del> <code>forkIO</code>: |
:{| |
:{| |
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|<haskell> |
|<haskell> |
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forkIO :: IO () -> IO ThreadId |
forkIO :: IO () -> IO ThreadId |
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− | forkIO act = do |
+ | forkIO act = do t <- unsafeInterleaveIO act |
case par t () of |
case par t () of |
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!_ -> do i' <- itsThreadId t |
!_ -> do i' <- itsThreadId t |
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Line 64: | Line 64: | ||
Can either of these prototypes ever go mainstream? |
Can either of these prototypes ever go mainstream? |
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* As shown by it's type signature, <code>par</code> is supposed to be pure: avoiding the use of <code>unsafeLocalState</code> means making it primitive; |
* As shown by it's type signature, <code>par</code> is supposed to be pure: avoiding the use of <code>unsafeLocalState</code> means making it primitive; |
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+ | * While the use of <code>unsafeInterleaveIO</code> may annoy some, it being one of the earlier Haskell extensions means it's widely available. |
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− | * Considering it's already <code>IO</code>-based, <code>forkIO</code> without <code>unsafeLocalState</code> seems more likely. |
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+ | For now, using a primitive <code>par</code> (and others) to define <code>forkIO</code> looks like the simplest option...but if using <code>unsafeInterleaveIO</code> ''really does'' annoy you, how about [[IO, partible-style|this]]: |
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− | [[IO, partible-style|This]] looks interesting: |
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+ | |||
− | <haskell> |
+ | :<haskell> |
forkIO :: (OI -> ()) -> OI -> ThreadId |
forkIO :: (OI -> ()) -> OI -> ThreadId |
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forkIO act u = let !(u1:u2:u3:_) = parts u in |
forkIO act u = let !(u1:u2:u3:_) = parts u in |
Revision as of 21:51, 20 April 2021
Parallelism vs concurrency: what's the difference?
Visible side effects.
- Have a look at this
ugly eysore"prototype definition" ofpar
:
par :: a -> b -> b par x y = case unsafeLocalState (forkIO (evalIO x >> return ())) of !_ -> y
where:
evalIO :: a -> IO a forkIO :: IO () -> IO ThreadId
Assuming:
x
is well-defined (it contains nounsafe...
calls),x
is well-behaved (not throwing exceptions or causing errors);
then:
forkIO
attaches aThreadId
to its argument, adds it to the work-queue and returns the identifier;par
then returnsy
;- Some time later,
forkIO
's argument is called, causingevalIO
to start evaluatingx
.
If
y
is still being evaluated when the evaluation ofx
commences, then we have elementary parallelism: concurrency, but with no visible side-effects.
- Now have a look at this
nearly-as-uglyprototype definition forspawnIO
forkIO
:
forkIO :: IO () -> IO ThreadId forkIO act = do t <- unsafeInterleaveIO act case par t () of !_ -> do i' <- itsThreadId t case i' of Just i -> return i Nothing -> ioError "forkIO"
where:
itsThreadId :: a -> IO (Maybe ThreadId)
Assuming:
par
anditsThreadId
are primitive,itsThreadId
would returnNothing
if it's argument had not been previously used bypar
;
then:
- Evaluating
par t ()
causes a newThreadId
to be attached tot
by the implementation; itsThreadId
retrievesi'
, the (possible) identifier fort
;forkIO
then extracts and returns the identifier.
This looks very much like elementary concurrency: parallelism, but having visible side effects.
Can either of these prototypes ever go mainstream?
- As shown by it's type signature,
par
is supposed to be pure: avoiding the use ofunsafeLocalState
means making it primitive; - While the use of
unsafeInterleaveIO
may annoy some, it being one of the earlier Haskell extensions means it's widely available.
For now, using a primitive par
(and others) to define forkIO
looks like the simplest option...but if using unsafeInterleaveIO
really does annoy you, how about this:
forkIO :: (OI -> ()) -> OI -> ThreadId forkIO act u = let !(u1:u2:u3:_) = parts u in let t = act u1 in case par t () of !_ -> case itsThreadId t u2 of Just i -> i Nothing -> ioError "forkIO" u3
-- Atravers Tue Apr 20 06:04:10 UTC 2021