once

memoization for IO actions and functions

GPL-3.0-only licensed by Dmitry Bogatov

Maintained by [email protected]

This version can be pinned in stack with:once-0.3@sha256:5fff6e0e1046eb12b780c1bdd06245d720c313adeff39e9bc8f48b57b0443389,2019

Depends on 5 packages(full list with versions):

Package provides single polymorphic function once, that allows you to memoize IO actions and functions, evaluating them at most once.

let mkStamp = (putStrLn "stamping" >> writeFile "/tmp/stamp" "") :: IO ()

-- onceStamp :: IO ()

onceStamp <- once mkStamp

-- onceStamp actually evaluates mkStamp it wraps first time.

onceStamp

stamping

-- but second time result `()' is memoized, no action is performed.

onceStamp

-- we can memoize functions too

foo <- once $ \x -> print "foo" >> print (x :: Int)

-- action will be performed once for every distinct argument

foo 10

foo

foo 10

foo 4

foo