Author: grant

Where to start with The Lambda Calculus

When you first start studying functional programming, one of the things that people will ask you is “So; have you learned lambda calculus (TLC)?”.
The fact is that while you don’t need to learn TLC to learn about functional programming; you ought too study the TLC at some point.
In this great LtU thread on “where to get started with studying programming language theory”, Anton van Straaten posted a comment referencing two posts on the PLT discussion list detailing a plan on what you should study when it comes to TLC and the texts that you should use to facilitate those studies.

Composing Functions with Scheme

In the PLT thread [‘complement'[?] of map] Stephen de Gabriel asked if there was a function that would take any number of functions, and an argument, and then apply the first function to the argument, and apply the second function to the result of the first, and so on.
In PLT Scheme, the function is called ‘compose’. Compose:

Returns a procedure that composes the given functions, applying the last f first and the first f last. The composed functions can consume and produce any number of values, as long as each function produces as many values as the preceding function consumes.

I asked if this was typical FP style, and Noel replied that it is so common that compose is an infix operation in Haskell and ML, as far as he could recall.
Here is an example (from the thread) of how it works using the “Pretty Big” language:

(define pam
  (lambda (datum . proc-list)
    ((apply compose proc-list) datum)))
(pam
 2
 (lambda (n) (/ n 7))
 (lambda (n) (- n 3))
 (lambda (n) (+ n 10))
 (lambda (n) (* n 7)))
> 3