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Version: edge


Tremor provides access to a growing number of functions that allow advanced data manipulation or access to additional information.

Functions are namespaced to make identification easier.

Tremor also supports user defined functions. There are a few noteworthy restrictions:

  1. Functions are pure / side effect free - you can not mutate event, state, or $ inside of a function.
  2. Functions have to return a value, as tremor-script is expression oriented.
  3. Functions can only be defined once, even if they take different forms or arguments. Function overloading is not supported.
  4. In matching functions, a default case is required.
  5. Functions can call other functions but they have to be a priori defined. The order of definitions is significant.
  6. Tail recursion is supported, and constrained to a maximum recursion depth. A recursion depth is imposed as tremor-script is designed to operate on infinite streams of data so indefinite blocking/recursion is not supportable by design.

Lets look at the types of functions we have.

Standard functions


Standard functions are the common user-defined functions. They take a given number of arguments, each with a name.


## This function adds two values together
fn add(a, b) with
a + b

Variable arguments

It is possible to use a variable number arguments by appending a final ... ellipsis in a standard function definition. The anonymous arguments will be provided via the args keyword.


Standard functions with a fixed arity, that is, without variable arguments, can be tail-recursive.

Tremor imposes a restriction in recursion depth. As tremor is an event processing system it is not desirable to have long running functions that block events from being processed through the system.

A function can recurse and call itself using the recur statement.


use std::array;

fn sum_(e, es) with
let l = array::len(es);
match l of
case l when l > 0 =>
let a = es[0];
recur(e + es[0], es[1:l])
case _ => e

fn sum(...) with
sum_(0, args)

Match functions


Since matching and extracting are a core functionality for tremor matching on function arguments is directly supported.

The same patterns that are used in match can be used in function cases including extractors. If any extracting pattern is used and matches the function argument will be replaced by the result of the extraction.

## calculates the fibonaci sequence
fn fib_(a, b, n) of
case (a, b, n) when n > 0 => recur(b, a + b, n - 1)
case _ => a

## calculates the fibonaci sequence
fn fib(n) with
fib_(0, 1, n)

Intrinsic Functions



Intrinsic functions cannot be user-defined.

Intrinsic functions represent builtin or pre-defined functions implemented in the rust programming language that are a builtin component of the tremor project and are provided out of the box.

The Standard library documentation, for example, is generated from the documentation provided in the Tremor Standard library. The Standard library is primarily composed of intrinsic or bulitin functions.