Pratt

Library for parsing expressions and a beautiful Kotlin DSL

Just define your operators and operands with the Kotlin DSL and the parser is ready!

Photographer: EmirVildanov

Example

We will parse simple mathematical expressions that contain

• Operands: lowercase latin letters 'a'..'z'
• Prefix unary operators: + and -
• Infix binary operators: +, -, '*' and right associative power operators ^
• Postfix unary operator: factorial !
• Parenthesis: ( and )

enum class TokenType {
    ID, PLUS, MULTIPLY, MINUS, POWER, BANG, LPAREN, RPAREN
}

fun getTokenType(char: Char) = when (char) {
    in 'a'..'z' -> ID
    '+' -> PLUS
    '*' -> MULTIPLY
    '-' -> MINUS
    '^' -> POWER
    '!' -> BANG
    '(' -> LPAREN
    ')' -> RPAREN
    else -> error("Unknown token")
}

val parser = pratt<Char, TokenType> {
    getTokenType = ::getTokenType

    atomic(ID)

    grouping(LPAREN to RPAREN)
    // prefix
    prefix(PLUS) precedence 3
    prefix(MINUS) precedence 3
    // infix
    infix(PLUS) precedence 1
    infix(MINUS) precedence 1
    infix(MULTIPLY) precedence 2
    infix(POWER) precedence 4 associativity RIGHT
    // postfix
    postfix(BANG) precedence 5
}

fun main() {
    // builds AST
    val tree = parser.parse("-(a+b)*c!".asIterable())
    println(tree)
}

Use as a dependency

• Go to the Releases page and find out the latest version
• Then add pratt as a Jitpack dependency

repositories {
    maven(url = "https://jitpack.io")
}

dependencies {
    implementation("com.github.Furetur:pratt:<latest version>")
}