Getting started with ANTLR: building a simple expression language

In this post, we will start working on a very simple expression language. We will build it in our language sandbox and therefore we will call the language Sandy.

I think that tool support is vital for a language: for this reason we will start with an extremely simple language but we will build rich tool support for it. To benefit from a language we need a parser, interpreters and compilers, editors and more. It seems to me that there is a lot of material on building simple parsers but very few material on building the rest of the infrastructure needed to make using a language practical and effective.

I would like to focus on exactly these aspects, making a language small but fully useful. Then you will be able to grow your language organically.

The code is available on GitHub: https://github.com/ftomassetti/LangSandbox. The code presented in this article corresponds to the tag 01_lexer_2021.

The language

The language will permit to define variables and expressions. We will support:

• integer and decimal literals
• variable definition and assignment
• the basic mathematical operations (addition, subtraction, multiplication, division)
• the usage of parenthesis

Examples of a valid file:

var a = 10 / 3
var b = (5 + 3) * 2 
var c = a / b

The tools we will use

We will use:

• ANTLR to generate the lexer and the parser
• use Gradle as our build system
• write the code in Kotlin. It will be very basic Kotlin, given I just started learning it.

Setup the project

Our build.gradle file will look like this

buildscript {
   ext.kotlin_version = '1.3.70'

   repositories {
     mavenCentral()
     maven {
        name 'JFrog OSS snapshot repo'
        url  'https://oss.jfrog.org/oss-snapshot-local/'
     }
     jcenter()
   }

   dependencies {
     classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
   }
}

apply plugin: 'kotlin'
apply plugin: 'java'
apply plugin: 'idea'
apply plugin: 'antlr'

repositories {
  mavenLocal()
  mavenCentral()
  jcenter()
}

dependencies {
  antlr "org.antlr:antlr4:4.9.1"
  compile "org.antlr:antlr4-runtime:4.9.1"
  compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
  compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
  testCompile "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
  testCompile "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"
  testCompile 'junit:junit:4.13'
}

generateGrammarSource {
    maxHeapSize = "64m"
    arguments += ['-package', 'me.tomassetti.langsandbox']
    outputDirectory = new File("generated-src/antlr/main/me/tomassetti/langsandbox".toString())
}
compileJava.dependsOn generateGrammarSource
sourceSets {
    generated {
        java.srcDir 'generated-src/antlr/main/'
    }
}
compileJava.source sourceSets.generated.java
compileKotlin.source sourceSets.generated.java
compileTestKotlin.source sourceSets.generated.java

clean{
    delete "generated-src"
}

idea {
    module {
        sourceDirs += file("generated-src/antlr/main")
    }
}

// show results when running gradlew test
test {
    testLogging {
        events "passed", "skipped", "failed"
    }
}

We can run:

• ./gradlew idea to generate the IDEA project files
• ./gradlew generateGrammarSource to generate the ANTLR lexer and parser
• ./gradlew test to run the tests (you have to generate the lexer and the parser before running this task)

Implementing the lexer

We will build the lexer and the parser in two separate files. This is the lexer:

lexer grammar SandyLexer;

// Whitespace
NEWLINE            : 'rn' | 'r' | 'n' ;
WS                 : [t ]+ ;

// Keywords
VAR                : 'var' ;

// Literals
INTLIT             : '0'|[1-9][0-9]* ;
DECLIT             : '0'|[1-9][0-9]* '.' [0-9]+ ;

// Operators
PLUS               : '+' ;
MINUS              : '-' ;
ASTERISK           : '*' ;
DIVISION           : '/' ;
ASSIGN             : '=' ;
LPAREN             : '(' ;
RPAREN             : ')' ;

// Identifiers
ID                 : [_]*[a-z][A-Za-z0-9_]* ;

Now we can simply run ./gradlew generateGrammarSource and the lexer will be generated for us from the previous definition.

Testing the lexer

Testing is always important but while building languages it is absolutely critical: if the tools supporting your language are not correct this could affect all possible programs you will build for them. So let’s start testing the lexer: we will just verify that the sequence of tokens the lexer produces is the one we aspect.

package me.tomassetti.sandy

import me.tomassetti.langsandbox.SandyLexer
import org.antlr.v4.runtime.CharStreams
import java.util.*
import kotlin.test.assertEquals
import org.junit.Test as test

class SandyLexerTest {

    fun lexerForCode(code: String) = SandyLexer(CharStreams.fromString(code))

    fun lexerForResource(resourceName: String) = SandyLexer(ANTLRInputStream(this.javaClass.getResourceAsStream("/${resourceName}.sandy")))

    fun tokens(lexer: SandyLexer): List<String> {
        val tokens = LinkedList<String>()
        do {
           val t = lexer.nextToken()
            when (t.type) {
                -1 -> tokens.add("EOF")
                else -> if (t.type != SandyLexer.WS) tokens.add(lexer.ruleNames[t.type - 1])
            }
        } while (t.type != -1)
        return tokens
    }

    @test fun parseVarDeclarationAssignedAnIntegerLiteral() {
        assertEquals(listOf("VAR", "ID", "ASSIGN", "INTLIT", "EOF"),
                tokens(lexerForCode("var a = 1")))
    }

    @test fun parseVarDeclarationAssignedADecimalLiteral() {
        assertEquals(listOf("VAR", "ID", "ASSIGN", "DECLIT", "EOF"),
                tokens(lexerForCode("var a = 1.23")))
    }

    @test fun parseVarDeclarationAssignedASum() {
        assertEquals(listOf("VAR", "ID", "ASSIGN", "INTLIT", "PLUS", "INTLIT", "EOF"),
                tokens(lexerForCode("var a = 1 + 2")))
    }

    @test fun parseMathematicalExpression() {
        assertEquals(listOf("INTLIT", "PLUS", "ID", "ASTERISK", "INTLIT", "DIVISION", "INTLIT", "MINUS", "INTLIT", "EOF"),
                tokens(lexerForCode("1 + a * 3 / 4 - 5")))
    }

    @test fun parseMathematicalExpressionWithParenthesis() {
        assertEquals(listOf("INTLIT", "PLUS", "LPAREN", "ID", "ASTERISK", "INTLIT", "RPAREN", "MINUS", "DECLIT", "EOF"),
                tokens(lexerForCode("1 + (a * 3) - 5.12")))
    }
}

Conclusions and next steps

We started with the first small step: we set up the project and built the lexer.

There is a long way in front of us before making the language usable in practice but we started. We will next work on the parser with the same approach: building something simple that we can test and compile through the command line.

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