scheme — consolidated language guide — DedSec Project

Scheme

Mental model

Scheme is a Lisp-family language emphasizing lexical scope, functions and a small core; particular standards and implementations vary.

Minimal source sample

(define (sum xs)
  (if (null? xs) 0 (+ (car xs) (sum (cdr xs)))))

Experiment to carry out

Trace the recursive list sum and prove that the empty list ends recursion; then test a singleton list.

A specific failure to distinguish

Scheme syntax and libraries vary by R5RS/R7RS and interpreter; avoid silently using implementation-specific extensions.

To evaluate the exercise, record the initial input, predicted behavior and actual observation in a compatible Scheme environment. Add a second run changing only one variable or argument; this makes the comparison attributable to a concrete cause. If the required runtime is not installed, treat the example as unexecuted source analysis and never label an imagined output as a passing test.

Scheme — Expanded practice workbook

R5RS-compatible exercise

Many Scheme implementations offer different extensions. Keep the core example in standard Scheme forms:

(define (first-result xs)
  (if (null? xs)
      (list 'empty)
      (list 'value (car xs))))
(display (first-result '(7 8)))
(newline)
(display (first-result '()))
(newline)

Expected structures are (value 7) and (empty). The tagged result is intentional: returning only #f as a missing-value sentinel could collide with a valid first item that actually equals #f. Compare (first-result '(#f 2)) to verify the distinction. Run with an R5RS-compatible mode, if necessary, and record the Scheme implementation and version.

Guard the domain before calling car

Calling car on an empty list violates its input contract. The conditional checks null? before accessing a head element. Make a deliberately broken function that calls car without checking, then capture the actual implementation-specific diagnostic. Some Scheme dialects provide extra exception constructs; do not pretend those are guaranteed by every R5RS interpreter.

Structural recursion experiment

Define a function that sums a proper list of numbers with (if (null? xs) 0 (+ (car xs) (sum-list (cdr xs)))). Prove that each call processes a shorter finite list and test negative numbers and an empty list. A dotted pair is not a proper list and can invalidate assumptions about cdr eventually returning the empty list.

Independent investigation

Compare equality using eq? and equal? for pairs and numbers; state which comparison matches the intended semantic contract. Avoid relying on incidental object identity for structural data. Document whether your interpreter conforms to R5RS or an extended dialect. Standard: https://standards.scheme.org/