Perl Contains the Lambda Calculus

(How to write a 163 line program to compute 1+1)

Length: 90 minutes

Prerequisites: None.

-Calculus (pronounced `lambda calculus') is a model of computation invented by Alonzo Church in 1934. It's analogous to Turing machines, but it's both simpler and more practical. Where the Turing machine is something like a model of assembly language, the -calculus is a model of function application. Like Turing machines, it defines a simplified programming language that you can write real programs in. Writing Turing machine programs is like writing in assembly language, but writing -calculus programs is more like writing in a higher-level language, because it has functions.

The two legal operations in the -calculus are to construct a function of one argument with a specified body, and to invoke one of these functions on an argument. What can be in the body of the function? Any legal expression, but expressions are limited to variables, function constructions, and function invocations. What can the argument be? It has to be another function; functions are all you have. With this tiny amount of machinery, we can construct a programming language that can express any computation that any other language can express.

Unlike most popular programming languages, Perl is powerful enough to express the -calculus directly, without the need to write a simulator. This means that if you want to try programming in the -calculus, you can do it directly in Perl, without having to implement a program to parse and evaluate -calculus expressions first. Perl's parser will parse -expressions, if you write them properly, and its evaluator will evaluate them.

In the -calculus, a function with formal parameter x and body B is denoted x.B. In Perl, we write sub { my $x = shift; B }.

To apply the function P to an argument Q, the usual -calculus notation is just (P Q). In Perl, we write $P->($Q).

The only expressions not of either of these two forms are simple variables. To apply the function v.B to some argument A is simple. The result is just B, but with any occurrences of v replaced with A instead. For example (x.(x (x y))(P Q)) reduces to ((P Q) ((P Q) y))---we replaced all the x's with (P Q)s.

By convention, function application is taken to be left-associative, so that (P Q R) is short for ((P Q) R).

From these few materials we can construct a programming system capable of performing arithmetic, constructing lists and trees, and expressing arbitrary recursive functions on these objects.

1. Perl and the Lambda Calculus
2. What is the Lambda Calculus?
3. Turing Machine
4. Turing Machines
5. Lambda-Calculus
6. Lambda-Calculus
7. Abstraction
8. Application
9. Application Example
10. Application
11. Abstraction Again
12. Goal
13. Values
14. Church-Rosser theorem
15. Undefined Values
16. Currying
17. Currying
18. Boolean Values
19. Boolean Value Example
20. Boolean Value Example
21. Data Structures
22. Ordered Pairs
23. Ordered Pair Example
24. Ordered Pair Example
25. Numbers
26. Number Example
27. Arithmetic
28. Recursion
29. Fixed Points
30. o For any _f_,
31. Y = IMG:lambda-6.gif:_f_.(IMG:lambda-6.gif:_x_.(_f_ (_x_ _x_)) IMG:lambda-6.gif:_x_.(_f_ (_x_ _x_)))
32. Return from Digression
33. OK, We're Done
34. Building an Interpreter
35. There's a Catch
36. Applicative Order Solution
37. Applicative Order Solution
38. IMG:lambda-4.gif:-Calculus in Perl

Additional Materials

• Runnable Perl source code:
  • Demonstration of recursive functions constructed with fixpoint operator. (Note: The punch line is at the end, so you might want to read it backwards.)
  • Demonstration of Church numeral arithmetic. (Note: Ditto.)
• If you're familiar with -calculus, but not with Perl, read the article that I submitted to the Journal of Functional Programming.
  • POD version
  • TeX version
  • DVI version
  • PostScript version
• If you're familiar with Perl, but not with the -calculus, read this other article instead. (Caution: Still in draft stage.)
  • HTML version
  • POD version

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