# Difference between revisions of "Currying"

Basvandijk (talk | contribs) |
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*<hask>("\t"++)</hask> is the "indent" function, |
*<hask>("\t"++)</hask> is the "indent" function, |
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*<hask>(`elem` "AEIOU")</hask> is the "is-capital-vowel-in-English" function (ignoring the "sometimes Y"). |
*<hask>(`elem` "AEIOU")</hask> is the "is-capital-vowel-in-English" function (ignoring the "sometimes Y"). |
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− | These are examples of [[partial application]] (and of |
+ | These are examples of [[partial application]] (and of a [[Section of an infix operator]]). |

Sometimes it's valuable to think about functions abstractly without specifically giving all their arguments: this is the [[Pointfree]] style. |
Sometimes it's valuable to think about functions abstractly without specifically giving all their arguments: this is the [[Pointfree]] style. |

## Revision as of 13:09, 31 May 2007

Currying is the process of transforming a function that takes multiple arguments into a function that takes just a single argument and returns another function if any arguments are still needed. In Haskell, *all* functions are considered curried: that is, *all functions in Haskell take just single arguments.*

```
div :: Int -> Int -> Int
```

`div 11 2`

unsurprisingly evaluates to `5`

.
But there's more that's going on than immediately meets the untrained eye. It's a two-part process. First, ```
div 11
```

*returns a function*of type

```
Int -> Int
```

`2`

, and yields `5`

.

```
Int -> Int -> Int
```

`Int`

s and returns an `Int`

", but what it's *really*saying is "takes an

`Int`

and returns something of the type `Int -> Int`

--that is, it returns a function that takes an `Int`

and returns an `Int`

. (One can write the type as `Int x Int -> Int`

if you really mean the former--but since all functions in Haskell are curried, that's not legal Haskell. Alternatively, using tuples, you can write `(Int, Int) -> Int`

, but keep in mind that the tuple constructor `(,)`

itself can be curried.)
Much of the time, currying can be ignored by the new programmer. The major advantage of considering all functions as curried is theoretical: formal proofs are easier when all functions are treated uniformly (one argument in, one result out). Having said that, there *are* Haskell idioms and techniques for which you need to understand currying.

Currying provides a convenient way of writing some functions without having to explicitly name them:

`(1+)`

(unsugared:`(+) 1`

) is the "increment" function,`(2*)`

is the "double" function,`("\t"++)`

is the "indent" function,`(`elem` "AEIOU")`

is the "is-capital-vowel-in-English" function (ignoring the "sometimes Y").

These are examples of partial application (and of a Section of an infix operator).

Sometimes it's valuable to think about functions abstractly without specifically giving all their arguments: this is the Pointfree style.

Sometimes half the work of the function can be done looking only at the first argument (but there really *is* only one argument, remember?): see functional dispatch.