racket-notes.md

Racket Notes

Notes on the metastasized Scheme implementation

• Every Racket file needs a language declaration at the top.

• Exact decimals (for currency) e.g. can be represented like so: #e.3. See also: https://xn--w5d.cc/2020/11/24/racket-decimal.html

• sort has to be given the sorting function in every instance.

• raco pkg install sicp gave me the SICP language pack.

• equal? is used for most non-numeric equality comparisons. = does numeric equality comparisons with most reasonable semantics. eq? only does object reference equality.

• sudo raco pkg install -i <package name(s)> is necessary for system-wide installation of Racket packages. (The default behavior is to install into /root when using sudo.)

• Example of partial function application in Racket: https://rosettacode.org/wiki/Partial_function_application#Racket

• The let* form can be used to reference values bound in other bindings.

• Note that remainder and modulo have different behavior.

• empty is an alias for '().

• In the context of a for statement, a positive integer can create a stream, like so:

  (for ([i 5])
      (display i)
      (newline))

• for* can be used for nested loops.

• Here's how to do a tail call-optimized factorial:

  (define (factorial n)
    (define (helper n acc)
      (if (= n 0)
          acc
          (helper (sub1 n) (* n acc))))
    (helper n 1))

• Here's the same thing, but memoized:

  (define (factorial)
    (let ([h (make-hash)])
      (define (factorial n)
        (cond
          [(= n 0) 1]
          [(hash-has-key? h n) (hash-ref h n)]
          [else
           (define (helper n acc)
             (if (= n 0)
                 acc
                 (helper (sub1 n) (* n acc))))
           (helper n 1)]))
      factorial))

• A memoized Fibonacci function:

  (define F
    (let ([f (make-hash)])
      (define (fib n)
        (cond
          [(<= n 1) n]
          [else
           (let ([fn (+ (fib (- n 1)) (fib (- n 2)))])
             (hash-set! f n fn)
             fn)]))
      fib))