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Grammar Language

The grammar language describes the syntax and structure of your language. The Langium grammar language is implemented using Langium itself and therefore follows the same syntactic rules as any language created with Langium. The grammar language will define the structure of the abstract syntax tree (AST) which in Langium is a collection of TypeScript types describing the content of a parsed document and organized hierarchically. The individual nodes of the tree are then represented with JavaScript objects at runtime.

In the following, we describe the Langium syntax and document structure.

Language Declaration

An entry Langium grammar file (i.e. a grammar which contains an entry rule) always starts with a header which declares the name of the language. For example, a language named MyLanguage would be declared with:

grammar MyLanguage

Every grammar file has a .langium extension and the entry grammar file needs to be referenced in langium-config.json. If you used the Yeoman generator to start your project, the configuration is already prepared.

Import of other grammar languages

It is possible to reuse grammar rules from other .langium files by importing them into your own grammar file.

import './path/to/an/other/langium/grammar';

This will import all grammar rules from the imported grammar file. It is therefore crucial to ensure that there are no duplicate rules between the different grammar files.

Contrary to entry grammars, imported grammars do not need to start with the keyword grammar.

Terminal Rules

The first step in parsing your language is lexing, which transforms a stream of characters into a stream of tokens. A token is a sequence of one or many characters which is matched by a terminal rule, creating an atomic symbol. The names of terminal rules are conventionally written in upper case.

The Langium parser is created using Chevrotain which has a built-in lexer based on Javascript Regular Expressions.

Langium also allows the use of Extended Backus-Naur Form (EBNF) Expressions for terminals, but we highly recommend that you write your terminals using Regular Expressions instead. EBNF expressions are internally translated by langium into Regular Expressions, as they are intended to allow porting Xtext grammars into Langium grammars – given their similarity.

With that said, both types of expressions can be used jointly in the same grammar.

The declaration of a terminal rule starts with the keyword terminal:

terminal ID: /[_a-zA-Z][\w_]*/;

Here, the token ID will match a stream of characters starting with the character _, a small letter, or a capital letter followed by a sequence of zero or many (cardinality *) alphanumeric characters (\w) or _.

The order in which terminal rules are defined is critical as the lexer will always return the first match.

Return Types

A terminal rule returns an instance of a TypeScript primitive type. If no return type is specified, the terminal rule will return a string by default.

terminal ID: /[_a-zA-Z][\w_]*/;
terminal INT returns number: /[0-9]+/;

Here, the terminal rule ID will return an instance of string while the terminal rule INT will return an instance of number.

The available return types in Langium are:

  • string
  • number
  • boolean
  • bigint
  • Date

Hidden Terminal Rules

The lexer tries to match every character in the document to a terminal rule or a keyword. It is therefore necessary to specify which characters or sequence of characters need to be ignored during lexing and parsing. Generally, you would want to ignore whitespaces and comments. This is achieved by adding the keyword hidden when defining a terminal rule. These hidden terminal rules are global and will be valid for all parser rules in the document.

hidden terminal WS: /\s+/;
hidden terminal ML_COMMENT: /\/\*[\s\S]*?\*\//;
hidden terminal SL_COMMENT: /\/\/[^\n\r]*/;

Parser Rules

While terminal rules indicate to the lexer what sequence of characters are valid tokens, parser rules indicate to the parser what sequence of tokens are valid. Parser rules lay the structure of objects to be created by the parser and result in the creation of the abstract syntax tree (AST) which represents the syntactic structure of your language. In Langium, parser rules are also responsible for defining the type of objects to be parsed.


A parser rule always starts with the name of the rule followed by a colon.

    'person' name=ID;

In this example, the parser will create an object of type Person. This object will have a property name which value and type must match the terminal rule ID (i.e. the property name is of type string and cannot start with a digit or special character).

By default, the parser will create an object with an inferred type corresponding to the parser rule name. It is possible to override this behavior by explicitly defining the type of the object to be created. This is done by adding the keyword returns followed by a separately declared type, or the keyword infers followed by the name of the type to be inferred for this rule (more about this in the next chapter):

Person infers OtherType:
    'person' name=ID;

The parser rule Person will now lead to the creation of objects of type OtherType instead of Person.

The Entry Rule

The entry rule is a parser rule that defines the starting point of the parsing step. The entry rule starts with the keyword entry and matches other parser rules.

entry Model:
    (persons+=Person | greetings+=Greeting)*;

In this example, the entry rule Model defines a group of alternatives. The parser will go through the input document and try to parse Person or Greeting objects and add them to the persons or greetings arrays, respectively. The parser reads the token stream until all inputs have been consumed.

Extended Backus-Naur Form Expressions

Parser rules are defined using Extended Backus-Naur Form-like (EBNF) expressions similar to the Xtext notation.


A cardinality defines the number of elements in a given set. Four different cardinalities can be defined for any expression:

  1. exactly one (no operator)
  2. zero or one (operator ?)
  3. zero or many (operator *)
  4. one or many (operator +)


Expressions can be put in sequence specifying the order they have to appear:

    'person' name=ID address=Address;

In this example, the rule Person must start with the person keyword followed by an ID token and an instance of the Address rule.


It is possible to match one of multiple valid options by using the pipe operator |. The already mentioned Model example specifies to parse either Person or Greeting, zero or many times (cardinality *):

entry Model:
    (persons+=Person | greetings+=Greeting)*;


Keywords are inline terminals which need to match a character sequence surrounded by single or double quotes, for example 'person' or "person". Keywords must not be empty and must not contain white space.


Assignments define properties on the type returned by the surrounding rule. There are three different ways to assign an expression (right side) to a property (left side).

  1. = is used for assigning a single value to a property.

        'person' name=ID

    Here, the property name will accept only one expression matching the terminal rule ID.

  2. += is used to assign multiple values to an array property.

        addresses+=STRING addresses+=STRING;

    Here, the array property addresses will accept two expressions matching the terminal rule STRING.

  3. ?= is used to assign a value to a property of type boolean. The value of the property of type boolean is set to true if the right part of the assignment is consumed by the parser.

        'employee' name=ID (remote?='remote')?

    Here the value of the property remote will be set to true if the keyword remote was successfully parsed as a part of the rule call. If the keyword remote is not consumed (cardinality is ?), the property remote is set to false.


With Langium, you can declare cross-references directly in the grammar. A cross-reference allows to reference an object of a given type. The syntax is:


The property will be a reference to an object of type Type identified by the token TOKEN. If the TOKEN is omitted, the parser will use the terminal or data type rule associated with the name assignment of the Type rule. If no such rule exists, then the token is mandatory.

    'person' name=ID;
    'Hello' person=[Person:ID] '!';

The Person in square brackets does not refer to the parser rule Person but instead refers to an object of type Person. It will successfully parse a document like:

person Bob
Hello Bob !

but the following:

person Bob
Hello Sara !

will result in an error message since the cross reference resolution will fail because a Person object with the name ‘Sara’ has not been defined, even though ‘Sara’ is a valid ID.

Unassigned Rule Calls

Parser rules do not necessarily need to create an object, they can also refer to other parser rules which in turn will be responsible for returning the object. For example, in the Arithmetics example:

	Definition | DeclaredParameter;

The parser rule AbstractDefinition will not create an object of type AbstractDefinition. Instead, it calls either the Definition or DeclaredParameter parser rule which will be responsible for creating an object of a given type (or call other parser rules if they are unassigned rule calls themselves).

In contrast, an assigned rule call such as parameter=DeclaredParameter means that an object is created in the current parser rule and assigns the result of the DeclaredParameter parser rule to the specified property parameter of that object.

Unordered Groups

In regular groups, expressions must occur in the exact order they are declared.

    'person' name=ID age=INT

Here a Person object needs to first declare the property name then age.

person Bob 25

will successfully be parsed to an object of type Person while

person 25 Bob

will throw an error.

However, it is possible to declare a group of properties in an unordered fashion using the & operator

    'person' name=ID & age=INT

will now allow name and age to be declared in any order.

person 25 Bob

will then successfully create an object of type Person.

Cardinality (?,*,+ operators) also applies to unordered group. Please note that assignments with a cardinality of + or * have to appear continuously and cannot be interrupted by an other assignment and resumed later.

Simple Actions

It is possible for a rule to return different types depending on declaration

interface TypeOne {
    name: string
RuleOne returns TypeOne:
    'keywordOne' name=ID | RuleTwo;

interface TypeTwo extends TypeOne {}
RuleTwo returns TypeTwo:
    'keywordTwo' name=ID;

A rule call is one of the ways to specify the return type. With more complex rules, the readability will be highly impacted. Actions allow to improve the readability of the grammar by explicitly defining the return type. Actions are declared inside of curly braces {}:

RuleOne returns TypeOne:
    'keywordOne' name=ID | {TypeTwo} 'keywordTwo' name=ID;

The example above requires that the return types TypeOne and TypeTwo are declared separately (see the next chapter). If the type returned by the action is created on-the-fly, the keyword infer needs to be added:

RuleOne infers TypeOne:
    'keywordOne' name=ID | {infer TypeTwo} 'keywordTwo' name=ID;

Now both TypeOne and TypeTwo are inferred from the rule definition. Note that we use the keyword infers (declarative) for the grammar rule, but infer (imperative) for the action.

Tree-Rewriting Actions

The parser is built using Chevrotain which implements a LL(k) parsing algorithm (left-to-right). Conceptually, a LL(k) grammar cannot have rules containing left recursion.

Consider the following:

    Addition '+' Addition | '(' Addition ')' | value=INT;

The parser rule Addition is left-recursive and will not be parseable. We can go around this issue by left-factoring the rule, i.e. by factoring out the common left-factor. We introduce a new rule SimpleExpression:

    SimpleExpression ('+' right=SimpleExpression)*;

    '(' Addition ')' | value=INT;

Unfortunately, left-factoring does not come without consequences and can lead to the generation of unwanted nodes. It is possible to “clean” the tree by using tree-rewriting actions.

Addition returns Expression:
    SimpleExpression ({Addition.left=current} '+' right=SimpleExpression)*;

    '(' Addition ')' | value=INT;

Essentially this means that when a + keyword is found, a new object of type Addition is created and the current object is assigned to the left property of the new object. The Addition then becomes the new current object. In imperative pseudo code it may look like this:

function Addition() {
    let current = SimpleExpression()
    while (nextToken == '+') {
        let newObject = new Addition
        newObject.left = current
        current = newObject
        current.right = SimpleExpression()

Please refer to this blog post for further details.

Data Type Rules

Data type rules are similar to terminal rules as they match a sequence of characters. However, they are parser rules and are therefore context-dependent. This allows for more flexible parsing, as they can be interspersed with hidden terminals, such as whitespaces or comments. Contrary to terminal rules, they cannot use regular expressions to match a stream of characters, so they have to be composed of keywords, terminal rules or other data type rules.

The following example from the domain model example uses the QualifiedName data type rule to enable references to other elements using their fully qualified name.

QualifiedName returns string:
    ID ('.' ID)*;

Data type rules need to specify a primitive return type.

Rule Fragments

If you are facing repetitive patterns in your grammar definition, you can take advantage of Rule Fragments to improve the grammar’s maintainability.

    'student' firstName=ID lastName=ID address=STRING phoneNumber=STRING grades=Grades;
    'teacher' firstName=ID lastName=ID address=STRING phoneNumber=STRING classes=Classes;
    'tech' firstName=ID lastName=ID address=STRING phoneNumber=STRING;

The parser rules Student, Teacher, and TechnicalStaff partly share the same syntax. If, for example, the assignment for phoneNumber had to be updated, we would need to make changes everywhere the phoneNumber assignment was used. We can introduce Rule Fragments to extract similar patterns and improve maintainability:

fragment Details:
    firstName=ID lastName=ID address=STRING phoneNumber=STRING;

    'student' Details grades=Grades;
    'teacher' Details classes=Classes;
    'tech' Details;

Fragment rules are not part of the AST and will therefore never create an object, instead they can be understood as being textually inserted where they are referenced.

Guard Conditions

It may be useful to group parser rules with small variations inside of a single parser rule. Given the following example:

entry Model:

RootElement infers Element:
    'element' name=ID '{'

    'element' name=ID '{'

The only difference between RootElement and Element is that the former has the boolean property isPublic. We can refactor the grammar so that only Element is present in the grammar with a guard condition that will determine which concrete syntax should be used by the parser:

entry Model:

	(<isRoot> isPublic?='public')? 
	'element' name=ID '{'

Element has the guard isRoot, which will determine whether the optional group containing the isPublic property is allowed to be parsed.

The entry rule Model sets the value of isRoot to true with element+=Element<true>, while isRoot is set to false inside of the Element<isRoot> parser rule with elements+=Element<false>.

In general, a guard condition on a group decides whether the parser is allowed to parse the group or not depending on the result of the evaluated condition. Logical operations can be applied, such as & (and), | (or) and ! (not) to fine-tune the exact conditions in which the group is supposed to be parsed.

Additionally, guard conditions can also be used inside of alternatives. See the following example:

entry Model:

	(<isRoot> 'root' | <!isRoot> 'element') name=ID '{'

The parser will always exclude alternatives whose guard conditions evaluate to false. All other alternatives remain possible options for the parser to choose from.

More Examples

Not all parser rules need to be mentioned in the entry rule, as shown in this example:

entry Model:
    (persons+=Person | greetings+=Greeting)*;

    'person' name=ID address=Address;

    'Hello' person=[Person] '!';

    street=STRING city=ID postcode=INT;

Here the Person parser rule includes a property address which matches the parser rule Address. We decided that an Address will never be present in the input document on its own and will always be parsed in relation to a Person. It is therefore not necessary to include an array of Address inside of the entry rule.

Keywords are meant to provide a visible structure to the language and guide the parser in deciding what type of object needs to be parsed. Consider the following:



    Student | Teacher;

In this example, a Person can either be a Student or a Teacher. This grammar is ambiguous because the parser rules Student and Teacher are identical. The parser will not be able to differentiate between the parser rules for Student and Teacher when trying to parse a Person. Keywords can help removing such ambiguity and guide the parser in defining if a Student or Teacher needs to be parsed. We can add a keyword to the parser rule Student, Teacher, or to both of them:

    'student' name=ID;

    'teacher' name=ID;

    Student | Teacher;

Now the ambiguity is resolved and the parser is able to differentiate between the two parser rules.

Parser rules can have many keywords:

    'person' name=ID 'age' age=INT;

If an assignment has a cardinality of + or *, then the expressions belong to a single group and must not be interrupted by other expressions.

    'paragraph' (sentences+=STRING)+ id=INT;

Here, the property sentences will accept one or many expressions matching the terminal rule STRING followed by an INT. The parsing of a document containing:

paragraph "The expression group " 3 "was interrupted"

will throw an error since the STRING expressions are not continuous. It is however possible to interrupt and resume a sequence of expressions by using hidden terminal symbols:

paragraph "expression one" /* comment */ "expression two" 3

The above example will be successfully parsed.

More on Terminal Rules

Extended Backus-Naur Form Terminals

For full disclosure, we recommend using regular expressions when writing your terminals, as EBNF expressions are translated to regular expressions internally anyway. EBNF support is primarily intended for supporting grammars that were originally written in Xtext, but are being ported to Langium.

As mentioned earlier, terminal rules can be described using regular expressions or EBNF expressions.

EBNF expressions are very similar to parser rules, which are described above. In this section, we describe which EBNF expressions are supported for terminals and their equivalent in Javascript Regular Expressions where possible.

Terminal Groups

Tokens can be put in sequence specifying the order they have to appear:

terminal FLIGHT_NUMBER: ('A'..'Z')('A'..'Z')('0'..'9')('0'..'9')('0'..'9')('0'...'9')?;

In this example, the token FLIGHT_NUMBER must start with two capital letters followed by three or four digits.

Terminal Alternatives

It is possible to match one of multiple valid options by using the pipe operator |. The terminal rule STRING can use alternatives to match a sequence of characters between double quotes "" or single quotes '':

terminal STRING: '"' !('"')* '"' | ''' !(''')* '''; 

In regular expression, alternatives are also possible with the pipe operator |:

terminal STRING: /"[^"]*"|'[^']*'/;

Character Range

The operator .. is used to declare a character range. It is equivalent to the operator - within a character class in a regular expression. It matches any character in between the left character and the right character (inclusive on both ends).

terminal INT returns number: ('0'..'9')+;

is equivalent to the regular expression:

terminal INT returns number: /[0-9]+/;

Here, INT is matched to one or more characters (by using the operand +, which defines a cardinality of ‘one or many’) between 0 and 9 (inclusive on both ends).

Wildcard Token

The operator . is used to match any character and is similar in regular expression.

terminal HASHTAG: '#'.+;

In this example, the terminal rule HASHTAG matches a sequence of character starting with # followed by one or many (cardinality +) characters.

Equivalent in regular expression:

terminal HASHTAG: /#.+/;

Until Token

The operator -> indicates that all characters should be consumed from the left token until the right token occurs. For example, the terminal rule for multi-line comment can be implemented as:

terminal ML_COMMENT: '/*' -> '*/';

Langium will transform the until token into the regular expression [\s\S]*? which matches any character non-greedily:

terminal ML_COMMENT: /\/\*[\s\S]*?\*\//;

Negated Token

It is possible to negate tokens using the operator !. In Langium this produces a negative lookahead. I.e., it does not consume tokens, but it is a ‘guard’ for what the following expression can recognize.

For example, if you want to recognize a word that doesn’t start with no, then you could write such an expression in EBNF like so:

terminal NONO: (!'no')('a'..'z'|'A'..'Z')+;

For reference, this would correspond to the following regular expression:

terminal NONO: /(?!no)[a-zA-Z]+/;

Note, if you’re coming from Xtext, negated tokens works differently here. In Xtext, negated tokens allow recognizing the complement of a set of characters (or anything ‘but’ what is listed in the negation), very much akin to a negated character class in regular expressions. This is very important to keep in mind if you’re porting a grammar from Xtext, as Langium’s interpretation of negated tokens deviates from that of Xtext.

Terminal Rule Calls

A terminal rule can include other terminal rules in its definition.

terminal DOUBLE returns number: INT '.' INT;

Note that it is easy to create conflicts between terminal rules when using terminal rule calls. See Data Type Rules for further details.

Terminal Fragments

Fragments allow for sub-definition of terminal rules to be extracted. They are not consumed by the lexer and have to be consumed by other terminal rules.

terminal fragment CAPITAL_LETTER: ('A'..'Z');
terminal fragment SMALL_LETTER: ('a'..'z');

In this example, the lexer will not transform a single capital or small letter into a valid token but will match a sequence of one capital letter followed by one or many small letters.