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+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`, `T`, `M` (aka ω
) and `L` 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.)
+
+*Variables*: Variables must start with a letter and can continue with any sequence of letters, numbers, `_`, `-`, or `/`. They may optionally end with `?` or `!`. When the evaluator does alpha-conversion, it may change `x` into `x'` or `x''` and so on. But you should not attempt to use primed variable names yourself.
+
+
+
+
+do eta-reductions too
+
+
+
+
+
+
++ + + + +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