# Difference between revisions of "Recursion in a monad"

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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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+ | |||

+ | <haskell> |
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+ | main = f 3 |
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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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+ | |||

+ | Runs: |
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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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+ | |||

+ | Or make it [[tail recursion|tail recursive]]: |
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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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+ | |||

+ | Or abstract the recursion pattern into a fold: |
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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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+ | |||

+ | where fn acc _ = do x <- getLine |
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+ | return (x:acc) |
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+ | </haskell> |
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+ | |||

+ | And finally, apply some functor and pointfree shortcuts: |
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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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+ | |||

+ | [[Category:Code]] |
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+ | [[Category:Glossary]] |

## Latest revision as of 15:18, 6 February 2021

People sometimes wonder how to effectively do recursion when inside a
monadic `do`

-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
```

Runs:

```
$ runhaskell A.hs
1
2
3
["1","2","3"]
```

Or make it 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
```