@lezer-unofficial/printer

This is a parse tree pretty printer for the lezer incremental parser.

Usage

1. Install this package.

   npm i @lezer-unofficial/printer

2. Generate a parser for your .grammar. I'll use @lezer/javascript.

3. Parse the code.

   import { parser } from "@lezer/javascript"
   import { printTree } from "@lezer-unofficial/printer"
   import { readFileSync } from "fs"
   
   const source = `
   function norm(a: number, b: number): number {
       return Math.sqrt(Math.pow(a, 2) + Math.pow(b, 2))
   }`
   const tree = parser.parse(source)

4. Print the tree with a built-in method.

   console.log(tree.toString())

   Output:

   Script(FunctionDeclaration(function,VariableDefinition,ParamList("(",VariableDefinition,TypeAnnotation(":",TypeName),",",VariableDefinition,TypeAnnotation(":",TypeName),")"),TypeAnnotation(":",TypeName),Block("{",ReturnStatement(return,CallExpression(MemberExpression(VariableName,".",PropertyName),ArgList("(",BinaryExpression(CallExpression(MemberExpression(VariableName,".",PropertyName),ArgList("(",VariableName,",",Number,")")),ArithOp,CallExpression(MemberExpression(VariableName,".",PropertyName),ArgList("(",VariableName,",",Number,")"))),")"))),"}")))

5. Print the tree with printTree from this package.

   console.log(printTree(tree, source))

   Output:

   Script [1:0..5:0]
   ┗━  FunctionDeclaration [2:0..4:1]
       ┣━  function [2:0..2:8]: "function"
       ┣━  VariableDefinition [2:9..2:13]: "norm"
       ┣━  ParamList [2:13..2:35]
       ┃   ┣━  ( [2:13..2:14]: "("
       ┃   ┣━  VariableDefinition [2:14..2:15]: "a"
       ┃   ┣━  TypeAnnotation [2:15..2:23]
       ┃   ┃   ┣━  : [2:15..2:16]: ":"
       ┃   ┃   ┗━  TypeName [2:17..2:23]: "number"
       ┃   ┣━  , [2:23..2:24]: ","
       ┃   ┣━  VariableDefinition [2:25..2:26]: "b"
       ┃   ┣━  TypeAnnotation [2:26..2:34]
       ┃   ┃   ┣━  : [2:26..2:27]: ":"
       ┃   ┃   ┗━  TypeName [2:28..2:34]: "number"
       ┃   ┗━  ) [2:34..2:35]: ")"
       ┣━  TypeAnnotation [2:35..2:43]
       ┃   ┣━  : [2:35..2:36]: ":"
       ┃   ┗━  TypeName [2:37..2:43]: "number"
       ┗━  Block [2:44..4:1]
           ┣━  { [2:44..2:45]: "{"
           ┣━  ReturnStatement [3:1..3:50]
           ┃   ┣━  return [3:1..3:7]: "return"
           ┃   ┗━  CallExpression [3:8..3:50]
           ┃       ┣━  MemberExpression [3:8..3:17]
           ┃       ┃   ┣━  VariableName [3:8..3:12]: "Math"
           ┃       ┃   ┣━  . [3:12..3:13]: "."
           ┃       ┃   ┗━  PropertyName [3:13..3:17]: "sqrt"
           ┃       ┗━  ArgList [3:17..3:50]
           ┃           ┣━  ( [3:17..3:18]: "("
           ┃           ┣━  BinaryExpression [3:18..3:49]
           ┃           ┃   ┣━  CallExpression [3:18..3:32]
           ┃           ┃   ┃   ┣━  MemberExpression [3:18..3:26]
           ┃           ┃   ┃   ┃   ┣━  VariableName [3:18..3:22]: "Math"
           ┃           ┃   ┃   ┃   ┣━  . [3:22..3:23]: "."
           ┃           ┃   ┃   ┃   ┗━  PropertyName [3:23..3:26]: "pow"
           ┃           ┃   ┃   ┗━  ArgList [3:26..3:32]
           ┃           ┃   ┃       ┣━  ( [3:26..3:27]: "("
           ┃           ┃   ┃       ┣━  VariableName [3:27..3:28]: "a"
           ┃           ┃   ┃       ┣━  , [3:28..3:29]: ","
           ┃           ┃   ┃       ┣━  Number [3:30..3:31]: "2"
           ┃           ┃   ┃       ┗━  ) [3:31..3:32]: ")"
           ┃           ┃   ┣━  ArithOp [3:33..3:34]: "+"
           ┃           ┃   ┗━  CallExpression [3:35..3:49]
           ┃           ┃       ┣━  MemberExpression [3:35..3:43]
           ┃           ┃       ┃   ┣━  VariableName [3:35..3:39]: "Math"
           ┃           ┃       ┃   ┣━  . [3:39..3:40]: "."
           ┃           ┃       ┃   ┗━  PropertyName [3:40..3:43]: "pow"
           ┃           ┃       ┗━  ArgList [3:43..3:49]
           ┃           ┃           ┣━  ( [3:43..3:44]: "("
           ┃           ┃           ┣━  VariableName [3:44..3:45]: "b"
           ┃           ┃           ┣━  , [3:45..3:46]: ","
           ┃           ┃           ┣━  Number [3:47..3:48]: "2"
           ┃           ┃           ┗━  ) [3:48..3:49]: ")"
           ┃           ┗━  ) [3:49..3:50]: ")"
           ┗━  } [4:0..4:1]: "}"

Background

There was a question on the CodeMirror forum about the best way to test and debug a grammar.

This example shows how to test a grammar.

For debugging, a user suggested printing the parse tree and provided the code.

I decided to package it because:

• Marijn isn't going to work on a pretty-printer.
• I found myself copying the gist into my second project.

Related works

• hast-util-from-lezer - Render styled Lezer syntax trees to hast.