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Functions

You can define your own custom functions in jq to encapsulate whatever logic you need. Functions act just like builtins: they take an input and emit zero, one or more outputs.

Defining a function

You can define a jq function using the following syntax:

# no arguments
def funcname: expression;

# or with arguments
def funcname(args): expression;

• starts with def keyword,
• a colon before the function body,
• the body consists of a single expression,
• ends with a semicolon,
• like the rest of jq syntax, you can use arbitrary whitespace for readability.

Where to put functions

Functions must be defined before they are used: this is an error:

def A: B(10);
def B(n): n + 1;
A
# => error: B/1 is not defined

This implies you have to place functions at the top of your jq code, prior to the "main" expression.

Nested functions

Functions can be nested:

def A:
    def B(n): n + 1;
    B(10)
    ;
A
# => 11

Here, the B function is only visible in the body of A.

Scope

A function introduces a new scope for variables and nested functions.

Arguments

Function arguments are separated by semi-colons not commas. For example, a function that takes a number, and then adds a number and multiplies by a number:

def add_mul(adder; multiplier): (. + adder) * multiplier;

10 | add_mul(5; 4)    # => 60

10 | add_mul(5, 4)
# error: add_mul/1 is not defined

Arguments are expressions

Function arguments are filters, not values. In this sense, they act like what other languages describe as callbacks:

Using the add_mul function as an example:

10 | add_mul(. + 5; . - 2)    # => 200

What's happening here?

• the adder argument gets the expression . + 5
  • when the function does . + adder, that becomes . + . + 5
  • that evaluates to 25 since . == 10
• similarly, the multiplier argument is the expression . - 2
  • that evaluates to 8
  • then the result is 25 * 8 == 200

Arguments as values

Sometimes you'll want to "materialize" an argument into a variable:

def my_func(arg):
    arg as $arg
    | other stuff ...
;

There's a shorthand for this:

def my_func($arg):
    other stuff ...
;

Take note that this is just "syntactic sugar": the name arg with no $ is still in scope in the function.

Arity

Functions have an arity -- the number of arguments they take.

Functions can use the same name with different arities. The builtin range function demonstrates this: range/1, range/2 and range/3 all co-exist.

This can be useful for defining recursive functions that carry state via arguments. For example map could be implemented like:

def my_map($accumulator; func):
    if length == 0
        then $accumulator
        else first as $elem | .[1:] | my_map($accumulator + [$elem | func]; func)
    end
    ;

def my_map(func):
    my_map([]; func)
    ;

[1, 2, 3, 4] | my_map(. * 10)   # => [10, 20, 30, 40]

Recursion

jq will perform tailcall optimization, but for 0-arity functions only.

Modules

A jq module is a file containing only functions. Modules are included into a jq program with the include or import commands.