@lezer-unofficial/printer

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

Usage

1. Install this package.

``console
npm i @lezer-unofficial/printer
`

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

1. Parse the code.

`ts
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)
`

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

`ts
console.log(tree.toString())
`

Output:

`console
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,")"))),")"))),"}")))
`

1. Print the tree with printTree from this package.

`ts
console.log(printTree(tree, source))
`

Output:

`console
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.