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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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| − | 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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