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Control.Parallel

Portability | non-portable
Stability | experimental
Maintainer | libraries@haskell.org

Description
Parallel Constructs
Synopsis

par :: a -> b -> b
pseq :: a -> b -> b
seq

Documentation

par :: a -> b -> b | Source

Indicates that it may be beneficial to evaluate the first argument in parallel with the second. Returns the value of the second argument.

`a `par` b` is exactly equivalent semantically to `b`.

`par` is generally used when the value of `a` is likely to be required later, but not immediately. Also it is a good idea to ensure that `a` is not a trivial computation, otherwise the cost of spawning it in parallel overshadows the benefits obtained by running it in parallel.

Note that actual parallelism is only supported by certain implementations (GHC with the `-threaded` option, and GPH, for now). On other implementations, `par a b = b`.

pseq :: a -> b -> b | Source

Semantically identical to `seq`, but with a subtle operational difference: `seq` is strict in both its arguments, so the compiler may, for example, rearrange `a `seq` b` into `b `seq` a `seq` b`. This is normally no problem when using `seq` to express strictness, but it can be a problem when annotating code for parallelism, because we need more control over the order of evaluation; we may want to evaluate `a` before `b`, because we know that `b` has already been sparked in parallel with `par`.

This is why we have `pseq`. In contrast to `seq`, `pseq` is only strict in its first argument (as far as the compiler is concerned), which restricts the transformations that the compiler can do, and ensures that the user can retain control of the evaluation order.

seq
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