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GHC provides two functions for controlling parallel execution, through
the ``Parallel'` interface:
interface Parallel where
infixr 0 `par`
infixr 1 `seq`
par :: a -> b -> b
seq :: a -> b -> b

The expression ``(x `par` y)'` *sparks* the evaluation of ``x'`
(to weak head normal form) and returns ``y'`. Sparks are queued for
execution in FIFO order, but are not executed immediately. At the
next heap allocation, the currently executing thread will yield
control to the scheduler, and the scheduler will start a new thread
(until reaching the active thread limit) for each spark which has not
already been evaluated to WHNF.
The expression ``(x `seq` y)'` evaluates ``x'` to weak head normal
form and then returns ``y'`. The ``seq'` primitive can be used to
force evaluation of an expression beyond WHNF, or to impose a desired
execution sequence for the evaluation of an expression.
For example, consider the following parallel version of our old
nemesis, ``nfib'`:
import Parallel
nfib :: Int -> Int
nfib n | n <= 1 = 1
| otherwise = par n1 (seq n2 (n1 + n2 + 1))
where n1 = nfib (n-1)
n2 = nfib (n-2)

For values of ``n'` greater than 1, we use ``par'` to spark a thread
to evaluate ``nfib (n-1)'`, and then we use ``seq'` to force the
parent thread to evaluate ``nfib (n-2)'` before going on to add
together these two subexpressions. In this divide-and-conquer
approach, we only spark a new thread for one branch of the computation
(leaving the parent to evaluate the other branch). Also, we must use
``seq'` to ensure that the parent will evaluate ``n2'` *before*
``n1'` in the expression ``(n1 + n2 + 1)'`. It is not sufficient to
reorder the expression as ``(n2 + n1 + 1)'`, because the compiler may
not generate code to evaluate the addends from left to right.

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