+This lambda evaluator will allow you to write lambda terms and evaluate (that is, normalize) them, and inspect the results.
+(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.)
+
+*Lambda terms*: lambda terms are written with a backslash, thus: `((\x (\y x)) z)`.
+
+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)`.
+
+*Let*: in order to make building a more elaborate set of terms easier, it is possible to define values using `let`.
+In this toy system, `let`s should only be used at the beginning of a file. If we have, for intance,
+
+ let true = (\x (\y x)) in
+ let false = (\x (\y y)) in
+ ((true yes) no)
+
+the result is `yes`.
+
+*Comments*: anything following a semicolon to the end of the line is ignored.
+Blank lines are fine.
+
+*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.
+
+*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.)
+
+
+
<textarea id="INPUT" style="border: 2px solid black; color: black; font-family: monospace; height: 3in; overflow: auto; padding: 0.5em; width: 100%;">
let true = \x y. x in
let false = \x y. y in
}());
</script>
+
+
+
+Under the hood
+---------------
+
+The interpreter is written in JavaScript and runs inside your browser.
+So if you decide to reduce a term that does not terminate (such as `((\x (x x)) (\x (x x)))`), it will be your
+browser that stops responding, not the wiki server.
+
+The main code is [here](http://lambda.jimpryor.net/code/lambda.js). Suggestions for improvements welcome.
+
+The code is based on:
+
+* Chris Barker's JavaScript lambda calculator
+* [Oleg Kiselyov's Haskell lambda calculator](http://okmij.org/ftp/Computation/lambda-calc.html#lambda-calculator-haskell).
+* The top-down JavaScript lexer and parser at <http://javascript.crockford.com/tdop/index.html>.
+
+Improvements we hope to add soon: the ability to reduce Combinatory Logic combinators and report the result as combinators, rather than in lambda forms.
+
+