# Automatic Concrete Syntax Tree Creation

Chevrotain has the capability to automatically create a concrete syntax tree (CST) during parsing. A CST is a simple structure which represents the entire parse tree. It contains information on every token parsed.

The main advantage of using the automatic CST creation is that it enables writing "pure" grammars. This means that the semantic actions are not embedded into the grammar implementation but are instead completely separated from it.

This separation of concerns makes the grammar easier to maintain and makes it easier to implement different capabilities on the grammar, for example: separate logic for compilation and for IDE support.

# AST vs CST

There are two major differences.

  1. An Abstract Syntax Tree would not normally contain all the syntactic information. This mean the exact original text can not always be re-constructed from the AST.

  2. An Abstract Syntax Tree would not represent the whole syntactic parse tree. It would normally only contain nodes related to specific parse tree nodes, but not all of those (mostly leaf nodes).

# Enabling

How to enable CST output?

This feature is enabled when a parser extends the CstParser class (opens new window).

import { CstParser } from "chevrotain"

class SelectParser extends CstParser {
  constructor() {
    super([])
  }
}

# Structure

The structure of the CST is very simple.

export type CstElement = IToken | CstNode
export type CstChildrenDictionary = { [elementName:string]:CstElement[] }

export interface CstNode {
    readonly name:string

    readonly children:CstChildrenDictionary

    readonly recoveredNode?:boolean
}

A single CstNode corresponds to a single grammar rule's invocation result.

$.RULE("qualifiedName", () => {})

input = ""

output = {
  name: "qualifiedName",
  children: {}
}

Each Terminal will appear in the children dictionary using the terminal's name as the key and an array of IToken as the value.

$.RULE("qualifiedName", () => {
  $.CONSUME(Identifier)
  $.CONSUME(Dot)
  $.CONSUME2(Identifier)
})

input = "foo.bar"

output = {
  name: "qualifiedName",
  children: {
    Dot: ["."],
    Identifier: ["foo", "bar"]
  }
}

Non-Terminals are handled similarly to Terminals except each item in the value's array Is the CstNode of the corresponding Grammar Rule (Non-Terminal).

$.RULE("qualifiedName", () => {
  $.SUBRULE($.singleIdent)
})

$.RULE("singleIdent", () => {
  $.CONSUME(Identifier)
})

input = "foo"

output = {
  name: "qualifiedName",
  children: {
    singleIdent: [
      {
        name: "singleIdent",
        children: {
          Identifier: ["foo"]
        }
      }
    ]
  }
}

Note that Terminals and Non-Terminals will only appear in the children object if they were actually encountered during parsing. This means that optional grammar productions may or may not appear in a CST node depending on the actual input, e.g:

$.RULE("variableStatement", () => {
  $.CONSUME(Var)
  $.CONSUME(Identifier)
  $.OPTION(() => {
    $.CONSUME(Equals)
    $.CONSUME(Integer)
  })
})

input1 = "var x"

output1 = {
  name: "variableStatement",
  children: {
    Var: ["var"],
    Identifier: ["x"]
    // no "Equals" or "Integer" keys
  }
}

input2 = "var x = 5"

output2 = {
  name: "variableStatement",
  children: {
    Var: ["var"],
    Identifier: ["x"],
    Equals: ["="],
    Integer: ["5"]
  }
}

# Extracting Alternatives to "sub" rules

So far the CST structure is quite simple, but how would a more complex grammar be handled?

$.RULE("statements", () => {
  $.OR([
    // let x = 5
    {
      ALT: () => {
        $.CONSUME(Let)
        $.CONSUME(Identifer)
        $.CONSUME(Equals)
        $.SUBRULE($.expression)
      }
    },
    // select age from employee where age = 120
    {
      ALT: () => {
        $.CONSUME(Select)
        $.CONSUME2(Identifer)
        $.CONSUME(From)
        $.CONSUME3(Identifer)
        $.CONSUME(Where)
        $.SUBRULE($.expression)
      }
    }
  ])
})

Some of the Terminals and Non-Terminals are used in both alternatives. It is possible to check for the existence of distinguishing terminals such as the "Let" and "Select". But this is not a robust approach.

let cstResult = parser.qualifiedName()

if (cstResult.children.Let !== undefined) {
  // Let statement
  // do something...
} else if (cstResult.children.Select !== undefined) {
  // Select statement
  // do something else.
}

Alternatively it is possible to refactor the grammar in such a way that both alternatives Would be completely wrapped in their own Non-Terminal "sub" rules.

$.RULE("statements", () => {
  $.OR([
    { ALT: () => $.SUBRULE($.letStatement) },
    { ALT: () => $.SUBRULE($.selectStatement) }
  ])
})

This is the recommended approach in this case as more and more alternations are added the grammar rule will become too difficult to understand and maintain due to verbosity.

# CstNodes Location

Sometimes the information regarding the textual location (range) of each CstNode is needed. This information is normally already present on the CstNodes nested children simply because the CstNode's children include the Tokens provided by the Lexer. However by default this information is not easily accessible as we would have to fully traverse a CstNode to understands its full location range information.

The feature for providing CstNode location directly on the CstNodes objects is available since version 4.7.0. Tracking the CstNodes location is disabled by default and can be enabled by setting the IParserConfig nodeLocationTracking (opens new window) to:

  • "full" (start/end for all offset/line/column)
  • or "onlyOffset", (start/end for only offsets)

for example:

import { CstParser } from "chevrotain"

class SelectParser extends CstParser {
  constructor() {
    super([], {
      nodeLocationTracking: "full"
    })
  }
}

Once this feature is enabled the optional location property (opens new window) on each CstNode would be populated with the relevant information.

Caveats

  • In order to track the CstNodes location every Token in the input Token vector must include its own location information.

    • This is enabled by default in the Chevrotain Lexer, See ILexerConfig.positionTracking (opens new window). However, if a third party Lexer is used in conjunction with a Chevrotain Parser, the Tokens produced by such a lexer must include the relevant location properties to allow the chevrotain parser to compute the CstNode locations.
  • A CstNode may be empty, for example when the matching grammar rule has not matched any token. In that case the default value for the location properties is NaN.

  • This feature has a slight performance and memory cost, this performance impact is linear and was measured at 5-10% for a full lexing + parsing flow. In general the more complex a grammar is (in terms of more CstNodes created per N tokens) the higher the impact. Additionally if the Parser has activated the error recovery capabilities of Chevrotain the impact would be at the high end of the given range, as the location tracking logic is more complex when some of the Tokens may be virtual/invalid.

# Fault Tolerance

CST output is also supported in combination with automatic error recovery. This combination is actually stronger than regular error recovery because even partially formed CstNodes will be present on the CST output and be marked using the recoveredNode" boolean property.

For example given this grammar and assuming the parser re-synced after a token mismatch at the "Where" token:

$.RULE("SelectClause", () => {
  $.CONSUME(Select)
  $.CONSUME2(Identifer)
  $.CONSUME(From)
  $.CONSUME3(Identifer)
  $.CONSUME(Where)
  $.SUBRULE($.expression)
})

// mismatch token due to typo at "wherrrre", parsing halts and re-syncs to upper rule so
// the suffix "wherrrre age > 25" is not parsed.
input = "select age from persons wherrrre age > 25"

output = {
  name: "SelectClause",
  children: {
    Select: ["select"],
    Identifier: ["age, persons"],
    From: ["from"]
    // No "Where" key d,ue to the parse error
    // No "expression" key due to the parse error
  },
  // This marks a recovered node.
  recoveredNode: true
}

This accessibility of partial parsing results means some post-parsing logic may be able to perform farther analysis. for example: offering auto-fix suggestions or provide better error messages.

# Traversing

So we now know how to create a CST and it's internal structure. But how do we traverse this structure and perform semantic actions? Some examples for such semantic actions:

  • Creation of an Abstract Syntax Tree (AST) to be later used in the rest of the compilation pipeline.
  • Running the input text in an interpreter, for example a Calculator's grammar and input can be evaluated to a numerical value.
  • Extracting specific pieces of information from the input.

One option would be to "manually" recursively "walk" the output CST structure.

// Tree Walker
export function toAst(cst) {
  const children = cst.children
  switch (cst.name) {
    case "selectStatement": {
      let columnsListCst = children.columnsList[0]
      let fromClauseCst = children.fromClause[0]

      let columnsListAst = toAst(columnsListCst)
      let fromClauseAst = toAst(fromClauseCst)

      return {
        type: "SelectStatementAst",
        columns: columnsListAst,
        from: fromClauseAst
      }
    }
    case "columnsList": {
      let columnName = children.identifier[0].image
      /*...*/
    }
    case "fromClause": {
      /*...*/
    }
    default: {
      throw new Error(
        `CST case handler not implemented for CST node <${cst.name}>`
      )
    }
  }
}

This is a valid approach, however it can be somewhat error prone:

  • No validation that the case names match the real names of the CST Nodes.
  • The validation for missing case handler (default case) depends on attempting to run toAst with invalid input. (Fail slow instead of fail fast...)

# CST Visitor

For the impatient, See a full runnable example: Calculator Grammar with CSTVisitor interpreter (opens new window)

Chevrotain provides a CSTVisitor class which can make traversing the CST less error prone.

// The base Visitor Class can be accessed via a Parser **instance**.
const BaseCstVisitor = myParserInstance.getBaseCstVisitorConstructor()

class SqlToAstVisitor extends BaseCstVisitor {
  constructor() {
    super()
    // This helper will detect any missing or redundant methods on this visitor
    this.validateVisitor()
  }

  selectStatement(ctx) {
    // ctx.columnsList is an array, while this.visit accepts a CSTNode
    // but if an array is passed to this.visit it will act as though the first element of the array has been passed.
    // this means "this.visit(ctx.columnsList)" is equivalent to "this.visit(ctx.columnsList[0])"
    let columnsListAst = this.visit(ctx.columnsList)
    let fromClauseAst = this.visit(ctx.fromClause)

    return {
      type: "SelectStatementAst",
      columns: columnsListAst,
      from: fromClauseAst
    }
  }

  columnsList(ctx) {
    let columnName = ctx.identifier[0].image
    /*...*/
  }

  // Optional "IN" argument
  fromClause(ctx, inArg) {
    /*...*/
  }
}
  • Each visitor method will be invoked with the respective CSTNode's children as the first argument (called ctx in the above example).

  • Recursively visiting None-Terminals can be accomplished by using the this.visit method. It will invoke the appropriate visit method for the CSTNode argument.

  • The this.visit method can also be invoked on an array on CSTNodes in that case It is equivalent to calling it on the first element of the input array.

  • Each visit method can return a value which can be used to combine the traversal results.

  • The this.validateVisitor() method can be used to detect missing or redundant visitor methods.

    • For example due to a refactoring of the grammar or a typo.
  • Visitor methods support an optional "IN" parameter.

# Do we always have to implement all the visit methods?

No, sometimes we only need to handle a few specific CST Nodes In that case use getBaseCstVisitorConstructorWithDefaults() to get the base visitor constructor. This base visitor includes a default implementation for all visit methods which simply invokes this.visit on all none terminals in the CSTNode's children.

// The base Visitor Class can be accessed via a Parser **instance**.
const BaseCstVisitorWithDefaults =
  myParserInstance.getBaseCstVisitorConstructorWithDefaults()

class SqlColumnNamesVisitor extends BaseCstVisitorWithDefaults {
  constructor() {
    super()
    this.result = []
    this.validateVisitor()
  }

  fromClause(ctx) {
    // collect only the names of the columns
    this.result.push(ctx.Identifier[0].image)
  }

  // All other visit methods will be "filled" automatically with the default implementation.
}

Note that when using a visitor with default visit implementations It is not possible to return values from the visit methods because the default implementation does not return any value, only traverses the CST thus the chain of returned values will be broken.

# Performance

On V8 (Chrome/Node) building the CST was measured at anywhere from 35%-90% of the performance versus a pure grammar's runtime (no output) depending on the grammar used. Particularly on its level of rules nesting.

This may be substantial yet please consider:

  • Chevrotain is already very fast (opens new window) So at worst at will degrade to just "fast"...

  • This comparison is not fair as a pure grammar that has no output also has very little use... The right comparison would be to versus embedding actions that built some alternative CST/AST output structure.

  • Parsing is usually just one step in a larger flow, so the overall impact even in the slower edge cases would be reduced.

It is therefore recommended to use the CST creation capabilities as its benefits (modularity / ease of maintenance) by far outweigh the costs (potentially reduced performance). except in unique edge cases.