# Simple inline functions

## Thin arrow notation

For brevity, in an inline function declaration, the keyword `function`

can be replaced
with the symbol `->`

. For example, `function($a, $b){$a + $b}`

can be written as
`->($a, $b){$a + $b}`

. The syntax is borrowed from Raku (formerly Perl 6), and is reminiscent
of lambda expressions in other languages:
`(a, b) -> {a + b}`

in Java, `(a, b) => {a + b}`

in C#.

The concise syntax is helpful when writing simple callbacks, for example `fold-left($seq, 1, ->($x, $y){$x*$y})`

to calculate the product of a sequence of numbers.

Argument types and return types can be included in the same way as with the full `function`

syntax,
though the benefits of conciseness typically apply only when these are omitted.

A zero-arity function can be written as, for example, `->(){doc('abc.xml')//z}`

.

## Context item functions

Simple inline functions taking a single argument can be simplified even further. For example,
the expression `->{@code}`

represents a function that takes a single argument (presumably an element node),
and returns a selected attribute of that node. A simple inline function takes a single argument with required type `item()`

,
and returns any sequence (type `item()*`

). The function body is evaluated with a singleton focus based
on the supplied argument value.

Simple inline functions are particularly convenient when providing functions as arguments to higher-order functions, many of which accept a single item as their one argument. For example, to sort employees in order of salary, you can write:

`sort(//employee, ->{@salary})`

Simple inline functions can access externally-defined local variables in the usual way (that is, they have a closure).

The expression `->{EXPR}`

is a syntactic shorthand for:

`function($x as item()) as item()* {$x!(EXPR)}`

## Underscore functions

For functions taking more than one argument, or an argument that is not confined to a single
item, an abbreviated syntax ("underscore functions") is provided. For example,
the expression `_{$1 + $2}`

is a function that takes two arguments and returns their sum.
It is equivalent to the expression:

`function($a1 as item()*, $a2 as item()*) as item()* {$a1 + $a2}`

The arity of an underscore function is determined by the highest-numbered parameter reference
appearing within the function. For example, `_{$2}`

is a function that takes two arguments
and returns the value of the second argument.

This syntax can also be used to define a zero-arity function, for example the following is a function that returns tomorrow's date:

`_{current-date() + xs:dayTimeDuration('P1D')}`