How Lipi works

Lipi is a tree-walking interpreter written in Go. It follows Writing an Interpreter in Go by Thorsten Ball, which builds a language called Monkey. Lipi swaps in Nepali keywords and error messages, and tracks line numbers on every token so each Galti can point at a line.

The code is split into small packages, one for each step:

lipi/
├── token/      the kinds of tokens and the Nepali keyword table
├── lexer/      text → tokens
├── ast/        the shapes of the syntax tree
├── parser/     tokens → tree (Pratt parser)
├── object/     runtime values and environments
├── evaluator/  tree → result, plus bhana and lambai
├── wasm/       the bridge used by the online playground
└── main.go     the lipi command

1. Lexer

The lexer reads the file one character at a time and groups characters into tokens. rakha naam = "Ram"; becomes:

RAKHA "rakha"   IDENT "naam"   ASSIGN "="   STRING "Ram"   SEMICOLON ";"

Each token also records its line and column. Words are checked against a small keyword table: rakha, kaam, farkau, yadi, natra, sahi and galat are keywords, and everything else is a name.

2. Parser

The parser turns the flat list of tokens into a tree. For 1 + 2 * 3:

      +
     / \
    1   *
       / \
      2   3

Lipi uses a Pratt parser. Each operator has a strength (* is stronger than +), and one loop keeps attaching operators as long as the next one is stronger than the current one. A function call is just another operator: ( after a value means “call it”, with the highest strength of all.

yadi and kaam are expressions in Lipi, so they are parsed the same way as 5 or -x, from the token that starts them.

3. Evaluator

The evaluator walks the tree and computes a value for each node. A + node evaluates its left side, then its right side, then adds them.

Variables are kept in an environment: a table of names and values with a link to the environment around it. Calling a function creates a new environment that links back to the one where the function was made. That link is what lets a closure remember its variables after the outer function has returned.

farkau wraps its value so it can pass up through nested blocks until it reaches the function call, where it is unwrapped. A Galti travels up the same way and stops the program.

4. Command line and browser

main.go reads a .lip file, runs it, and prints any Galti. The wasm/ package compiles the same interpreter to WebAssembly and exposes one function, runLipi(code), to JavaScript. The playground on this site calls it, so the browser runs exactly the same code as your terminal.