In computer science, **partial application** (or **partial function application**) refers to the process of fixing a number of arguments of a function, producing another function of smaller arity. Given a function , we might fix (or 'bind') the first argument, producing a function of type . Evaluation of this function might be represented as . Note that the result of partial function application in this case is a function that takes two arguments. Partial application is sometimes incorrectly called currying, which is a related, but distinct concept.

Intuitively, partial function application says "if you fix the first arguments of the function, you get a function of the remaining arguments". For example, if function *div*(*x*,*y*) = *x*/*y*, then *div* with the parameter *x* fixed at 1 is another function: *div*_{1}(*y*) = *div*(1,*y*) = 1/*y*. This is the same as the function *inv* that returns the multiplicative inverse of its argument, defined by *inv*(*y*) = 1/*y*.

The practical motivation for partial application is that very often the functions obtained by supplying some but not all of the arguments to a function are useful; for example, many languages have a function or operator similar to `plus_one`

. Partial application makes it easy to define these functions, for example by creating a function that represents the addition operator with 1 bound as its first argument.

In languages such as ML, Haskell and F#, functions are defined in curried form by default. Supplying fewer than the total number of arguments is referred to as partial application.

In languages with first-class functions, one can define `curry`

, `uncurry`

and `papply`

to perform currying and partial application explicitly. This might incur a greater run-time overhead due to the creation of additional closures, while Haskell can use more efficient techniques.^{[1]}

Scala implements optional partial application with placeholder, e.g. `def add(x: Int, y: Int) = {x+y}; add(1, _: Int)`

returns an incrementing function. Scala also supports multiple parameter lists as currying, e.g. `def add(x: Int)(y: Int) = {x+y}; add(1) _`

.

Clojure implements partial application using the `partial`

function defined in its core library.^{[2]}

The C++ standard library provides `bind(function, args..)`

to return a function object that is the result of partial application of the given arguments to the given function. Since C++20 the function `bind_front(function, args...)`

is also provided which binds the first `sizeof...(args)`

arguments of the function to the args. In contrast, `bind`

allows binding any of the arguments of the function passed to it, not just the first ones. Alternatively, lambda expressions can be used:

```
int f(int a, int b);
auto f_partial = [](int a) { return f(a, 123); };
assert(f_partial(456) == f(456, 123) );
```

In Java, `MethodHandle.bindTo`

partially applies a function to its first argument.^{[3]}
Alternatively, since Java 8, lambdas can be used:

```
public static <A, B, R> Function<B, R> partialApply(BiFunction<A, B, R> biFunc, A value) {
return b -> biFunc.apply(value, b);
}
```

In Raku, the `assuming`

method creates a new function with fewer parameters.^{[4]}

The Python standard library module `functools`

includes the `partial`

function, allowing positional and named argument bindings, returning a new function.^{[5]}

In XQuery, an argument placeholder (`?`

) is used for each non-fixed argument in a partial function application.^{[6]}

In the simply-typed lambda calculus with function and product types (*λ*^{→,×}) partial application, currying and uncurrying can be defined as

`papply`

- (((
*a*×*b*) →*c*) ×*a*) → (*b*→*c*) =*λ*(*f*,*x*).*λy*.*f*(*x*,*y*) `curry`

- ((
*a*×*b*) →*c*) → (*a*→ (*b*→*c*)) =*λf*.*λx*.*λy*.*f*(*x*,*y*) `uncurry`

- (
*a*→ (*b*→*c*)) → ((*a*×*b*) →*c*) =*λf*.*λ*(*x*,*y*).*f x y*

Note that `curry`

`papply`

= `curry`

.

Partial application can be a useful way to define several useful notions in mathematics.

Given sets and , and a function , one can define the function

where is the set of functions . The image of under this map is . This is the function which sends to . There are often structures on which mean that the image of restricts to some subset of functions , as illustrated in the following examples.

A group action can be understood as a function . The partial evaluation restricts to the group of bijections from to itself. The group action axioms further ensure is a group homomorphism.

An inner-product on a vector space over a field is a map . The partial evaluation provides a canonical map to the dual vector space, . If this is the inner-product of a Hilbert space, the Riesz representation theorem ensures this is an isomorphism.

The partial application of the cross product on is . The image of the vector is a linear map such that . The components of can be found to be .

This is closely related to the adjoint map for Lie algebras. Lie algebras are equipped with a bracket . The partial application gives a map . The axioms for the bracket ensure this map is a homomorphism of Lie algebras.