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| People sometimes wonder how to effectively do recursion when inside a
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| monadic <hask>do</hask>-block. Here's some quick examples:
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| The problem is to read 'n' lines from stdin, recursively:
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|
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| The obvious, recursive way:
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|
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| <haskell>
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| main = f 3
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|
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| f 0 = return []
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| f n = do v <- getLine
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| vs <- f (n-1)
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| return $! v : vs
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| </haskell>
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|
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| Runs:
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|
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| <haskell>
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| $ runhaskell A.hs
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| 1
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| 2
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| 3
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| ["1","2","3"]
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| </haskell>
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|
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| Or make it [[tail recursion|tail recursive]]:
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|
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| <haskell>
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| f 0 acc = return (reverse acc)
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| f n acc = do
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| v <- getLine
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| f (n-1) (v : acc)
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| </haskell>
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|
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| Or abstract the recursion pattern into a fold:
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|
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| <haskell>
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| f n = do
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| s <- foldM fn [] [1..n]
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| return (reverse s)
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|
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| where fn acc _ = do x <- getLine
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| return (x:acc)
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| </haskell>
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|
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| And finally, apply some functor and pointfree shortcuts:
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|
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| <haskell>
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| f n = reverse `fmap` foldM fn [] [1..n]
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| where fn acc _ = (: acc) `fmap` getLine
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| </haskell>
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|
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| [[Category:Code]]
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| [[Category:Glossary]]
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