# Recursion in a monad

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Revision as of 15:18, 6 February 2021 by Gwern (talk | contribs) (Reverted edits by Tomjaguarpaw (talk) to last revision by Lemming)

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
```