Difference between revisions of "Recursion in a monad"

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(link to tail recursion)
(Deleting page that hasn't been edited for over 10 years)
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People sometimes wonder how to effectively do recursion when inside a
monadic <hask>do</hask>-block. Here's some quick examples:
The problem is to read 'n' lines from stdin, recursively:
The obvious, recursive way:
main = f 3
f 0 = return []
f n = do v <- getLine
vs <- f (n-1)
return $! v : vs
$ runhaskell A.hs
Or make it [[tail recursion|tail recursive]]:
f 0 acc = return (reverse acc)
f n acc = do
v <- getLine
f (n-1) (v : acc)
Or abstract the recursion pattern into a fold:
f n = do
s <- foldM fn [] [1..n]
return (reverse s)
where fn acc _ = do x <- getLine
return (x:acc)
And finally, apply some functor and pointfree shortcuts:
f n = reverse `fmap` foldM fn [] [1..n]
where fn acc _ = (: acc) `fmap` getLine

Revision as of 14:19, 6 February 2021