Skip to main content
Version: 0.12


This section will give you an overview over all the expressions available in Tremor, be it Select statements, Script or Connector definitions.


Comments in Tremor are single-line comments that begin with a '#' symbol and continue until end of line.

# I am a comment

Doc Comments

Doc comments are used for producing documentation for entities in Tremor. They consist of a single line beginning with '##' token and continue until end of line. Multiple consecutive lines will be combined to form a documentation text.

## I am a doc-comment, documenting the const below
const FALSE = true;

Module Doc Comments

Module doc comments are used for producing documentation for Module files in Tremor. They consist of a single line beginning a ### token and continue until the end of line. Multiple consecutive lines will be combined to forma module documentation text.

### I am a module doc comment
### This is my third line, cool innit?


Literal in Tremor are equivalent to their sibling types supported by the JSON format. In fact, any well-formed JSON document is a valid Tremor literal.


The null literal which represents the absence of a defined value



Boolean literal.


Integer Numerics

Integers in Tremor are signed and are limited to 64-bit internal representation


The stdlib provides useful function for integers in std::integer.

use std::integer;
integer::parse("42") == 42

Floating-Point Numerics

Floating point numerics in Tremor are signed and are limited to 64-bit IEEE representation


The stdlib provides useful function for floats in std::float.

Character and Unicode Code-points

The language does not support literal character or Unicode code-points at this time.

UTF-8 encoded Strings

"I am a string"

The standard library provides useful function for string manipulation in std::string:

use std::string;
string::uppercase(string::substr("snotty", 0, 4)) == "SNOT"

String Interpolation

For strings tremor allows string interpolation, this means embedding code directly into strings to create strings out of them.

"I am a #{ "string with #{1} interpolation." }"

A hash sign followed by a curly bracket needs to be escaped \#{ hash signs themselves do not need to be escaped.


To deal with pre formatted strings in tremor script we allow for heredocs they are started by using triple quotes """ that terminate the line (aka """bla isn't legal). Heredocs do not truncate leading indentation, only the first leading linebreak after the leading triple-quote """ stripped.

I am
string with #{ "#{1} interpolation" }

Since Tremor 0.9 Heredocs also support String Interpolation. A hash sign followed by a curly bracket needs to be escaped \#{ hash signs themselves do not need to be escaped.


Array grammar:

array grammar

Array literals in Tremor are a comma-delimited set of expressions bracketed by the square brackets '[' and ']'.

[ 1, 2, "foobar", 3.456e10, { "some": "json-like-document" }, null ]

The standard library provides several useful functions to work with arrays in std::array:

use std::array;

array::push(["snot"], "badger") == ["snot", "badger"]


Record grammar:

record grammar

Field grammar:

field grammar

Record literals in Tremor are syntactically equivalent to JSON document objects

"field1": "value1",
"field2": [ "value", "value", "value" ],
"field3": { "field4": "field5" }

Check out the stdlib std::record module for some helpful function for working with records.


Binaries are based on the Erlang bit syntax.

Binary grammar:

binary literal grammar

Bytes grammar:

binary segments grammar

Bytes Part

field grammar

Parts of each field are: <value>:<size>/<type> where both size and type are optional. Without size or type, the field defaults to an unsigned big endian integer with 8 bits (aka 1 byte).

The binary types consists of up to three parts. That is 2 prefixes and 1 main type identifier. Examples: unsigned-big-integer, signed-integer, binary. The types currently supported are:

  • binary - this can handle both binaries and strings, size here refers to the number of bytes
  • integer - this can represent integers, size here means size in bits. In addition the type can be prefixed with big and little for indianness and signed and unsigned for signedness.

Some examples would be:

  • <<1:1, 42:7>>
  • <<(1 + 1)/unsigned-big-integer>>
  • <<1:4, "badger"/binary, -2:4/signed-little-integer>>

We could construct a TCP package this way:

# constructing a TCP package
# using made up, non-correct values

let event = {
"src": {"port": 1234},
"dst": {"port": 2345},
"seq": event,
"ack": 4567,
"offset": 1,
"res": 2,
"flags": 3,
"win": 4,
"checksum": 5,
"urgent": 6,
"data": "snot badger!"

event.src.port:16, event.dst.port:16,
event.offset:4, event.res:4, event.flags:8,,
event.checksum:16, event.urgent:16,

See also:


List of binary and unary operators in Tremor, ordered by precedence (from low to high):

orLogical ORtrue or falsebool
andLogical ANDtrue and falsebool
|Bitwise ORBitwise OR has not been implemented yet-
^Bitwise XOR42 ^ 42, true ^ trueinteger, bool
&Bitwise AND42 & 0, true & falseinteger, bool
==, !=Equality, Inequality"snot" != "badger"all
<, <=, >, >=Comparison Operators42 > 0integer, float, string, binary
<<, >>, >>>Bitwise shift -- Left, Right(signed), Right(unsigned)42 >> 2integer
+, -Addition, Subtraction42 + 0integer, float, string
*, /, %Multiplication, Division, Modulus42 * 1integer, float (no modulo)
+, -Unary Plus, Unary Minus+42integer, float, string
not , !Unary Logical NOT, Unary Bitwise NOTnot false, Bitwise NOT has not been implemented yetbool

These should not be confused with the higher level Operators that are part of a Pipeline.


Path grammar:

path grammar

Path Segments grammar:

qualified segment grammar

ArraySegment grammar:

array grammar

Path-like structures in Tremor allow referencing local variables, ingested events, event meta-data, script-local state etc. and also indexing into them if they are records or arrays.

Reserved paths

Normal paths are used to referring to local variables created with let, but Tremor offers a set of reserved paths used to refer to commonly needed entities:

  • event: Always referring to the currently handled event.
  • $: Referring to the event metadata. Values inside the event metadata can only be accessed via a top-level name like: $udp.port. Its contents are usually either null or a record.
  • state: Referring to the script's state, which will persist across the lifetime of a pipeline, but not across tremor reboots. So it can be used as state kept across different events. Default value is null.
  • args: Referring to a record of arguments passed into the script definition or create script.
  • window: Referring to the name of the window this event is emitted from. This is null if the event is not handled inside a select statement with a window.
  • group: Referring to the current group if the event is handled inside a select statement with a group by clause. It will be null outside of a group by select, if used inside, it will be an array where the first element is the value of the current group, and the second element is the stringified name of the group, derived from the group value.


Example event for illustration purposes:

"store": {
"book": [
"category": "reference",
"author": "Nigel Rees",
"title": "Sayings of the Century",
"price": 8.95
"category": "fiction",
"author": "Herman Melville",
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"price": 8.99
"category": "fiction",
"author": "J.R.R. Tolkien",
"title": "The Lord of the Rings",
"isbn": "0-395-19395-8",
"price": 22.99
"bicycle": {
"color": "red",
"price": 19.95
"expensive": 10

Grab the entire event document:

let capture = event;

Grab the books from the store (the same using key, index and escaped key notation for field lookup):

let capture =;
# index and escaped notation can accommodate keys that include 'odd' characters such as whitespaces or dots.
let capture =["book"];
let capture =`book`;

Grab the first book:

let capture =[0];

Grab the title of the 3rd book:

let capture =[2].title

Grab the range of books from 0 ( the first ) to 2 ( the last ), exclusive of the last book:

let capture =[0:2];

The type of a path is equivalent to the type of the data returned by a path expression. So in the above examples, a reference to a book title would return the value at that path, which in the reference event document is a string.

Path's in Tremor are themselves expressions in their own right.


Const grammar:

const grammar

Const can be used to define immutable, constant values that get evaluated at compile time. This is more performant than let as all logic can happen at compile time and is helpful for setting up lookup tables or other never changing data structures.


Let grammar:

let grammar

The let expression allows data pointed to by a path to be destructively mutated, and the pointed-to value reassigned. If the path does not yet exist, it will be created in-situ:

Set a local variable a to the literal integer value 10:

let a = 10;

Set a local variable a to be the ingested event record

let a = event;

Set the metadata variable a to be the value of the local variable a:

let $a = a;


Drop expressions enable short-circuiting the evaluation of a Script when badly formed data is discovered. If no argument is supplied, drop will return the event record. If an argument is supplied, the result of evaluating the expression will be returned. Tremor or other processing tools can process dropped events or data using purpose-built error-handling.

As the content of the dropped event is user-defined, operators can standardise the format of the error emitted on drop from Tremor

drop; # As the first drop always wins, this expression never runs


Emit grammar:

emit grammar

Emit expressions enable short-circuiting the evaluation of a Script when processing is known to be complete and further processing can be avoided. If no argument is supplied, emit will return the event record. If an argument is supplied, the result of evaluating the expression will be returned. Tremor or other processing tools can process emitted events or data using their default flow-based or stream-based data processing pipelines.

As the content of the emitted event is user-defined, operators can standardise the format of the event emitted on emit from a Script.


By default, if no emit or drop expressions are defined, all expressions in a correctly written Script will be evaluated until completion and the value of the last expression evaluated will be returned as an emit message.

Implicit emission:

"badgers" # implicit emit

Explicit emission of "snot":

"badgers" # literals do not short-circuit processing, so we continue to the next expression in this case
emit "snot"
emit "oh noes!"
emit "never happens"; # As the first emit always wins, this expression never runs

There are times when it is necessary to emit synthetic events from a Script within a tremor pipeline to an alternate operator port than the default success route. For example, when data is well-formed but not valid and the data needs to be diverted into an alternate flow. The emit clause can be deployed for this purpose by specifying an optional named port.

emit {
"event": event,
"status": "malformed",
"required field `loglevel` is absent"
} => "invalid";


Match grammar:

match grammar

Match case grammar:

case grammar

Match expressions enable data to be filtered or queried using case-based reasoning. Match expressions take the form:

match <target> of
case <case-expr> [ <guard> ] => <block>
default => <block>


  • target: An expression that is the target of case-based queries
  • case-expr: A predicate test, literal value or pattern to match against
  • guard: An optional predicate expression to gate whether or not an otherwise matching case-clause will in fact match
  • block: The expressions to be evaluated if the case matches, and any supplied guard evaluates to true


Discover if the path is an array, record or scalar structure:

match of
case %[] =>
let msg = " is an array-like data-structure",
case %{} => " is a record-like data-structure"
default => " is a scalar data-type"

Find all fiction books in the store:

let found = match of
case fiction = %[ %{ category ~= "fiction" } ] => fiction
default => []
emit found;

Matching literal expressions

The simplest form of case expression in match expressions is matching a literal value. Values can be any legal Tremor type and they can be provided as literals, computed values or path references to local variables, metadata or values arriving via events.

let example = match 12 of
case 12 => "matched"
default => drop "not possible"
let a = "this is a";
let b = " string";
let example = match a + b of
case "this is a string" => "matched"
default => drop "not possible"
let a = [ 1, "this is a string", { "record-field": "field-value" } ];
match a of
case a => a
default => drop "not possible"

Matching on test predicate expressions

It is also possible to perform predicate based matching

match "this is not base64 encoded" of
case ~ base64|| => "surprisingly, this is legal base64 data"
default => drop "as suspected, this is not base64 encoded"

These are often referred to informally as tilde expressions and tremor supports a variety of micro-formats that can be used for predicate or test-based matching such as logstash dissect, json, influx, perl-compatible regular expressions.

Tilde expressions can under certain conditions elementize ( extract ) micro-format data. The elementization or extraction is covered in the Extractors section of this document and in the Extractor reference.

Match and extract expressions

It is also possible to elementize or ingest supported micro-formats into Tremor for further processing. For example, we can use the ~= and ~ operator to perform a predicate test, such as the base64 test in the previous example, which upon success, extracts ( in the base64 case, decoding ) a value for further processing.

For example if we had an embedded JSON document in a string, we could test for the value being well-formed json, and extract the contents to a local variable as follows:

let sneaky_json = "
{ \"snot\": \"badger\" }

match sneaky_json of
case json = ~ json|| => json
default => drop "this is not the json we were looking for"

Matching tuple patterns


A tuple pattern matches a target value if the target is an array and each test matches the positionally correspondent value in the target. The target needs to be at least as long as the pattern but can be longer if the pattern ends with ....

If you are looking for a more set like operation look at the array pattern.

tuple case grammar

Tuple Pattern filter grammar:

tuple filter grammar

In addition to literal array matching, where the case expression tuple literal must exactly match the target of the match expression one for one, tuple patterns enable testing for matching elements within an array and filtering on the basis of matched elements.

let a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
match a of
case %( 0 ) => "is a zero"
case %( 0, .. ) => "starts with a zero"
case %( _, 1, .. ) => "has 1 one at index 1"
default => "does not contain zero's"

Matching array patterns


An array pattern matches a target value if the target is an array and each test in the pattern matches at least for one element in the target indiscriminate of their positions.

If you are looking for a more array like / positional operation look at the tuple pattern.

array case grammar

Array Pattern filter grammar:

array filter grammar

In addition to a subset match, where the elements of the pattern must be included in the target of the match expression, array patterns enable testing for matching elements within an array and filtering on the basis of matched elements.

let a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
match a of
case %[ 0 ] => "contains zero's"
default => "does not contain zero's"

Predicate matching against supported micro-formats is also supported in array pattern matching.

let a = [ "snot", "snot badger", "snot snot", "badger badger", "badger" ];
match a of
case got = %[ ~re|^(P<hit>snot.*)$| ] => got
default => "not snotty at all"

Matching record patterns


A record pattern matches a target if the target is a record that contains at least all declared keys and the tests for each of the declared key match.

record case grammar

Record Pattern Fields grammar

record pattern field grammar

Similarly to record literal matching where the case expression record must exactly match the target of the match expression, record patterns enable testing for matching fields or sub-structures within a record and extracting and elementizing data on the basis of matched predicate tests ( via ~= ).

We can check for the presence of fields:

match { "superhero": "superman", "human": "clark kent" } of
case %{ present superhero, present human } => "ok"
default => "not possible"

We can check for the absence of fields:

match { "superhero": "superman", "human": "clark kent" } of
case %{ absent superhero, absent human } => "not possible"
default => "ok"

We can test the values of fields that are present:

match { "superhero": "superman", "human": "clark kent" } of
case %{ superhero == "superman" } => "we are saved! \o/"
case %{ superhero != "superman" } => "we may be saved! \o/"
default => "call 911"

We can test for records within records:

match { "superhero": { "name": "superman" } } of
case %{ superhero ~= %{ present name } } => "superman is super"
case %{ superhero ~= %{ absent name } } => "anonymous superhero is anonymous"
default => "something bad happened"

We can also test for records within arrays within records tersely through nested pattern matching:

match { "superhero": [ { "name": "batman" }, { "name": "robin" } ] } of
case id = %{ superhero ~= %[ %{ name ~= re|^(?P<kind>bat.*)$|} ] } => id
default => "something bad happened"

Guard clauses

guard clause grammar

Guard expressions in Match case clauses enable matching data structures to be further filtered based on predicate expressions. For example they can be used to restrict the match to a subset of matching cases where appropriate.

match event of
case record = %{} when record.log_level == "ERROR" => "error"
default => "non-error"


Effectors grammar:

effectors grammar


block grammar

Effectors are the expressions evaluated when a case pattern and guard succeeded. When we have to use multiple expressions to do some more complex processing, we need to separate those expressions with commas ,:

use std::string;
match event of
case record = %{ present foo } =>
let foo_content = record["foo"],
let replaced = string::replace(foo_content, "foo", "bar"),
let record["foo"] = replaced
default => null


merge grammar

Merge expressions define a difference against a targeted record and applies that difference to produce a result record. Merge will result in an error if any of the operands are not records.

Merge rules

The table below is read with each row replacing the variables in the following statement:

let result = merge given of merge end;
{"a":"b"}{"a":"c"}{"a":"c"}Insert/Update field 'a'
{"a":"b"}{"b":"c"}{"a":"b", "b":"c"}Insert field 'b'
{"a":"b","b":"c"}{"a":null}{"a":null,"b":"c"}Set field 'a' to 'null'
{"a": [{"b":"c"}]}{"a": [1]}{"a": [1]}Replace field 'a' with literal array


let event = merge event of {"some": "record"} end;


patch grammar

Patch operation grammar

patch operation grammar

Patch expressions define a set of record level field operations to be applied to a target record in order to transform a targeted record. Patch allows fields to be: inserted where there was no field before; removed where there was a field before; updated where there was a field before; or inserted or updated regardless of whether or not there was a field before. Patch also allows field level merge operations on records or for the targeted document itself to be merged. Merge operations in patch are syntax sugar in that they are both based on the merge operation.

Patch follows the semantics of RFC 6902 with the explicit exclusion of the copy and move operations and with the addition of an upsert operation the variant supported by Tremor

let foo = {"foo":"bar"}patch foo of insert "baz" => "qux" end{"foo":"bar","baz":"qux"}Add baz field
let foo = {"foo":"bar","baz":"qux"}patch foo of erase "foo" end{"baz":"qux"}Erase foo and add baz field
let foo = {"foo":"bar"}patch foo of upsert "foo" => null end{"foo":null}Set foo to null, or reset to null if field already exists

For comprehensions

for grammar

For Case Clause grammar

for case clause grammar

For expressions are case-based record or array comprehensions that can iterate over index/element or key/value pairs in record or array literals respectively.

Given our book store example from above:

let wishlist = for of
case (i,e) =>
for e of of
case (k,v) when k == "price" and v > 20.00 => { "title": e.title, "isbn": e.isbn }
default => {}


As part of the tremor pipeline processing, there are times when it's necessary to track state across events over time (eg: in order to exploit stateful algorithms for session tracking, or building and maintaining application state). For this purpose, a tremor pipeline is equipped with operator node-level state management and storage capabilities that persists for the running lifetime of a pipeline deployed into the tremor runtime.

From a Script, this shared storage is accessible via the state keyword, which allows for accessing the storage contents via path expressions, akin to how the event keyword works (with the key difference being that the state storage is shared across events). On pipeline initialization, the state is initialized as null and users are free to set it to arbitrary value over the course of processing.

Here's a Script example demonstrating the usage of the state keyword -- it maintains a counter for the events coming in and emits the count alongside the event:

define script with_state
match type::is_null(state) of
case true =>
let state = {"count": 1}
default =>
let state.count = state.count + 1

"count": state.count,
"event": event

A key thing to note is that by design, state is not shared across operator nodes in the pipeline. Therefore, if we have scripts across multiple nodes in the pipeline, the state keyword in each script allows access only to the local node-specific state storage, and not the state from any other operator nodes or something global to all the nodes.

Since the state storage lives for the lifetime of a pipeline, state will not be persisted when the pipeline is undeployed or the main process is shut down.


test expression grammar


test literal grammar

The language has pluggable support for a number of microformats with two basic modes of operation that enable predicate tests ( does a particular value match the expected micro-format ) and elementization ( if a value does match a specific micro-format, then extract and elementize accordingly ).

The general form of a supported micro-format is as follows:



  • name - The key for the micro-format being used for testing or extraction
  • format - An optional multi-line micro-format specific format encoding used for testing and extraction

Formats can be spread out over multiple lines by adding a \ as a last character of the line. Spaces at the start of the line will be truncated by the lowest number of leading spaces. So if 3 lines respectively have 2, 4, and 7 spaces then 2 spaces are going to be removed from each line leaving 0, 2, and 5 spaces at the start.

The set of supported micro-formats at the time of writing is available in the Extractors Reference