Par and seq

The functions

par

and

seq

have much in common.

They have the same signature and thus the same use patterns.

The function

par

let you start a computation in parallel and

seq

forces a computation to actually take place (avoiding lazy evaluation).

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

You could also define a function

par2

which computes the arguments of a two-argument function

f

in parallel before feeding them to

f

par2 :: (a -> b -> c) -> (a -> b -> c)
par2 f x y = x `par` y `par` f x y

The function

par2

is also universal, because you can define

par

in terms of

par2

par = par2 (\ _x y -> y)

You can see, that

par

is the more basic combinator, because it scales to any number of arguments.

See also