1 This lambda evaluator will allow you to write lambda terms and evaluate (that is, normalize) them, and inspect the results.
2 (This won't work in Racket, because Racket doesn't even try to represent the internal structure of a function in a human-readable way.)
4 *Lambda terms*: lambda terms are written with a backslash, thus: `((\x (\y x)) z)`.
6 If you click "Normalize", the system will try to produce a normal-form lambda expression that your original term reduces to (~~>). So `((\x (\y x)) z)` reduces to `(\y z)`.
8 *Let*: in order to make building a more elaborate set of terms easier, it is possible to define values using `let`.
9 In this toy system, `let`s should only be used at the beginning of a file. If we have, for intance,
11 let true = (\x (\y x)) in
12 let false = (\x (\y y)) in
17 *Comments*: anything following a semicolon to the end of the line is ignored.
20 *Abbreviations*: In an earlier version, you couldn't use abbreviations. `\x y. y x x` had to be written `(\x (\y ((y x) x)))`. We've upgraded the parser though, so now it should be able to understand any lambda term that you can.
22 *Constants*: The combinators `S`, `K`, `I`, `C`, `B`, `W`, and `T` are pre-defined to their standard values. Also, integers will automatically be converted to Church numerals. (`0` is `\s z. z`, `1` is `\s z. s z`, and so on.)
26 <textarea id="INPUT" style="border: 2px solid black; color: black; font-family: monospace; height: 3in; overflow: auto; padding: 0.5em; width: 100%;">
28 let false = \x y. y in
29 let and = \l r. l r false in
31 (and true true yes no)
32 (and true false yes no)
33 (and false true yes no)
34 (and false false yes no)
37 <input id="PARSE" value="Normalize" type="button">
38 <input id="ETA" type="checkbox">do eta-reductions too
39 <noscript><p>You may not see it because you have JavaScript turned off. Uffff!</p></noscript>
40 <script src="/code/lambda.js"></script>
41 <script src="/code/tokens.js"></script>
42 <script src="/code/parse.js"></script>
43 <script src="/code/json2.js"></script>
47 /*jslint evil: true */
49 /*members create, error, message, name, prototype, stringify, toSource,
53 /*global JSON, make_parse, parse, source, tree */
55 // Make a new object that inherits members from an existing object.
57 if (typeof Object.create !== 'function') {
58 Object.create = function (o) {
65 // Transform a token object into an exception object and throw it.
67 Object.prototype.error = function (message, t) {
69 t.name = "SyntaxError";
76 var parse = make_parse();
79 var string, tree, expr, eta;
82 // string = JSON.stringify(tree, ['key', 'name', 'message', 'value', 'arity', 'first', 'second', 'third', 'fourth'], 4);
83 expr = tree.handler();
84 // string = JSON.stringify(expr, ['key', 'name', 'message', 'value', 'arity', 'first', 'second', 'tag', 'variable', 'left', 'right', 'bound', 'body' ], 4);
85 // string = expr.to_string() + "\n\n~~>\n\n";
87 eta = document.getElementById('ETA').checked;
88 string = string + reduce(expr, eta, false).to_string();
90 string = JSON.stringify(e, ['name', 'message', 'from', 'to', 'key',
91 'value', 'arity', 'first', 'second', 'third', 'fourth'], 4);
93 document.getElementById('OUTPUT').innerHTML = string
94 .replace(/&/g, '&')
95 .replace(/[<]/g, '<');
98 document.getElementById('PARSE').onclick = function (e) {
99 go(document.getElementById('INPUT').value);
110 The interpreter is written in JavaScript and runs inside your browser.
111 So if you decide to reduce a term that does not terminate (such as `((\x (x x)) (\x (x x)))`), it will be your
112 browser that stops responding, not the wiki server.
114 The main code is [here](http://lambda.jimpryor.net/code/lambda.js). Suggestions for improvements welcome.
116 The code is based on:
118 * Chris Barker's JavaScript lambda calculator
119 * [Oleg Kiselyov's Haskell lambda calculator](http://okmij.org/ftp/Computation/lambda-calc.html#lambda-calculator-haskell).
120 * The top-down JavaScript lexer and parser at <http://javascript.crockford.com/tdop/index.html>.
122 Improvements we hope to add soon: the ability to reduce Combinatory Logic combinators and report the result as combinators, rather than in lambda forms.