lambster

A lambda calculus interpreter written in TypeScript, packaged for npm.

Features

• Clean, minimal command-line interface with REPL and ability to run files

• No external dependencies

• Selectable verbosity for lambda calculus term reduction output:

  • Result (none): only the result of the reduction

    λ> (Lx y. x y)(Lw. (Lx.x w) a w) b
    >>> ((a b) b)
    

  • Reductions (low): intermediate reductions

    λ> (Lx y. x y)(Lw. (Lx.x w) a w) b
    λ > (((λx. (λy. (x y))) (λw. (((λx. (x w)) a) w))) b)
    β > (((λx. (λy. (x y))) (λw. ((a w) w))) b)
    β > ((λy. ((λw. ((a w) w)) y)) b)
    β > ((λy. ((a y) y)) b)
    β > ((a b) b)
    >>> ((a b) b)
    

  • Step-by-step (high): intermediate reductions with plain English explanation for each reduction

    λ> (Lx y. x y)(Lw. (Lx.x w) a w) b
    λ > (((λx. (λy. (x y))) (λw. (((λx. (x w)) a) w))) b)
    
    Beta reducing 'a' into '(λx. (x w))'
    β > (((λx. (λy. (x y))) (λw. ((a w) w))) b)
    
    Beta reducing '(λw. ((a w) w))' into '(λx. (λy. (x y)))'
    β > ((λy. ((λw. ((a w) w)) y)) b)
    
    Beta reducing 'y' into '(λw. ((a w) w))'
    β > ((λy. ((a y) y)) b)
    
    Beta reducing 'b' into '(λy. ((a y) y))'
    β > ((a b) b)
    
    >>> ((a b) b)
    

• Simple variable system with useful predefined lambda calculus terms

  λ> duplicate = Lx.x x
  >>> duplicate = (λx. (x x))
  λ> duplicate z
  >>> (z z)
  

  λ> env
  true:	(λt. (λf. t))
  false:	(λt. (λf. f))
  and:	(λa. (λb. ((a b) a)))
  or:	(λa. (λb. ((a a) b)))
  not:	(λb. ((b false) true))
  if:	(λp. (λa. (λb. ((p a) b))))
  ... (all bindings in the environment are printed)
  λ> or true false
  >>> (λX0. (λf. X0))
      ↳ equivalent to: true
  

• Optional free variable renaming

  λ> (Ly.y) y
  λ > ((λy. y) y)
  ε > 'y' → 'X`0'
  β > X`0
  >>> X`0
  

• Shorthand for introducing nested lambda functions

  λ> (Lx y. y x)
  >>> (λx. (λy. (y x))
  

• Shorthand for introducing lambda functions

  λ> (lambda x. x) y
  >>> y
  λ> (λx. x) y
  >>> y
  λ> (Lx. x) y
  >>> y
  

• Clear and robust error printing

  λ> (xx
  Error at line 1 [4, 5]: Expected ')' to close expression.
      (xx
         ^
  λ> Lx x
  Error at line 1 [5, 6]: Expected dot after abstraction declaration, got '<newline>'.
      Lx x
          ^
  

• Results are checked for structural equivalence to terms bound in the environment

  λ> env
  plus:	(λm. (λn. ((m incr) n)))
  one:	(λf. (λx. (f x)))
  two:	(λf. (λx. (f (f x))))
  three:	(λf. (λx. (f (f (f x)))))
  ...
  λ> plus one two
  >>> (λf. (λy. (f (f (f y)))))
      ↳ equivalent to: three
  

• Simple comment support

  λ> (Lx.x) y # This is a comment
  >>> y
  

Installation

lambster is available on npm! Simply install the package:

npm i lambster

Usage

CLI

After lambster is installed, launch the interactive prompt:

$ lambster

Client

After lambster is installed, import and use the interpreter:

import { Interpreter, Verbosity, InterpreterOptions } from "lambster";

const interpreter: Interpreter = new Interpreter();

interpreter.evaluate("(Ly. y) z");
// <console output>
// >>> z

The interpreter can also be passed an InterpreterOptions to control output parameters.

Interpreter output is surfaced via console.log by default, and can be customized via the transports option.

import { Interpreter, Verbosity, InterpreterOptions } from "lambster";

let output: string = "";

const interpreter: Interpreter = new Interpreter({
    verbosity: Verbosity.LOW,
    transports: [
        log => (output += log), // called once for each step of the reduction
    ],
});

interpreter.evaluate("(Lx.x x) y");

console.log(output);
// <console output>
// λ > ((λx. (x x)) y)
// β > (y y)
// >>> (y y)

API Reference

class Interpreter:

• constructor(options?: InterpreterOptions)

  • Create an instance of an Interpreter, optionally with the given options.
  • If options are not provided, interpreter parameters are given these default values:
    • verbosity: Verbosity.NONE (prints result only)
    • transports: [console.log]
    • rename_free_vars: false
    • show_equivalent: true

• interpret(source: string)

  • Interpret the given string and write the output to the provided transports

interface InterpreterOptions

• verbosity?: Verbosity
  • Optional Verbosity level for the interpreter
• transports?: LoggerTransport[]
  • Optional list of transport functions that the interpreter will pass its output to
• rename_free_vars?: boolean
  • Optional boolean that controls whether the interpreter will rename free variables to unambiguous names
• show_equivalent?: boolean
  • Options boolean that controls whether the interpreter will also print named terms that the result of a reduction is structurally equivalent to

enum Verbosity

• NONE
  • Indicates the least amount of verbosity to the interpreter. Only the result of the reduction will be logged.
• LOW
  • Indicates that the interpreter will log the result and type of each intermediate reduction while reducing a given term.
• HIGH
  • Indicates that the interpreter will log a plain English explanation along with each intermediate reduction while reducing a given term.

Grammar

For all you PL nerds out there, the interpreter recognizes this grammar:

Grammar for lambster

program     → ( stmt )* EOF ;

stmt        → bindingStmt
            | commandStmt
            | termStmt
            | "\n" ;
bindingStmt → IDENTIFIER "=" expression "\n" ;
commandStmt → "help" "\n"
            | "env" "\n"
            | "unbind" IDENTIFIER "\n" ;
termStmt    → term "\n" ;
term        → IDENTIFIER
            | abstraction
            | application
            | grouping ;
abstraction      → LAMBDA IDENTIFIER+ "." expression ;
application → expression expression ;
grouping    → "(" expression ")" ;
LAMBDA      → "L" | "\" | "λ" | "lambda" ;
IDENTIFIER  → [a-z0-9]+ ;

Modified for recursive descent parsing

term            → abstraction
                | application ;
abstraction     → LAMBDA IDENTIFIER+ "." term ;
application     → atom atom* ;
atom            → "(" term ")"
                | IDENTIFIER
                | abstraction ;

Operator precedence (descending)

Name	Associates
Application	Left
Abstraction	Extends as far to the right as possible

Planned features

• Automatically parse numeric constants into their Church numeral equivalent
• Support for De Bruijn indices
• Even prettier printing in step-by-step verbosity
• Library functions that do useful/interesting things (TBD)