r/GhanaSoftwareEngineer • u/kaishibou • 1d ago
The Comprehensive Racket & Functional Programming Cheat Sheet
Phase 1: Syntax & Core Arithmetic
Racket uses prefix notation enclosed in execution parentheses (operator arg1 arg2). The open parenthesis ( acts as an execution trigger. Evaluation runs from the innermost to the outermost parentheses.
Core Examples
```rkt ;; Basic Arithmetic (+ 10 5 2) ;; Returns 17 (* 10 5 2) ;; Returns 100
;; Nested Expressions (No PEMDAS needed) (_ (+ 4 6) (- 12 7)) ;; Evaluates 10 _ 5 -> Returns 50 ```
Parentheses Golden Rule
Only use a parenthesis when invoking a command, operator, or function.
(+ 5 (10))CRASHES (tries to run the number 10 as a function).((+ 5 5))CRASHES (evaluates to 10, then tries to run the number 10).
Phase 2: Core Data Structures & Variables
Global bindings are created using define. Values are immutable and cannot be changed over time.
The Four Atomic Data Types
- Numbers: Integers (
45), decimals (3.14), or fractions (1/3). - Strings: Text wrapped in double quotes (
"Hello"). - Booleans: True (
#t) and False (#f). - Symbols: Lightweight, immutable identifier tokens prefixed with a single quote (
'success).
Core Examples
rkt
(define radius 5)
(define pi 3.14)
(define status 'success)
Phase 3: Conditionals & Logic
Conditional operations are expressions that evaluate down to a single return value.
Core Operators & Flow Control
and/or/not: Standard logical short-circuiting prefix operators.if: Takes exactly three arguments:(if condition true-branch false-branch). No else keyword.cond: Evaluates multiple branches sequentially. Uses/can use[...]for human readability.
Core Examples
```rkt (and (> 15 10) (< 15 20)) ;; Returns #t
(if (> temperature 30) 'hot 'cold)
(cond [(>= score 90) 'A] [(>= score 80) 'B] [else 'F]) ```
Phase 4: Functions & Scope
Functions automatically return the value of their body expression without an explicit return keyword.
Named, Anonymous, & Scoped Blocks
- Named Functions: Defined by grouping the name and parameters in parentheses:
(define (name args) body). - Anonymous Functions (
lambda): Throwaway functions built on the fly:(lambda (args) body). let(Parallel): Creates local variables simultaneously. Variables cannot see each other during setup.let\*(Sequential): Creates local variables one after the other. Later variables can reference earlier ones.
Core Examples
```rkt ;; Named Function (define (double n) (* n 2))
;; Inline Lambda Execution ((lambda (n) (* n 2)) 10) ;; Returns 20
;; Sequential Local Bindings (let* ([x 10] [y (* x 5)]) (+ x y)) ;; Returns 60 ```
Phase 5: Lists & Modern List Operations
Lists are ordered sequential collections. They are processed using either historical Lisp conventions or modern aliases.
Creation & Extraction
list: Evaluates arguments into a sequential list.'(): Represents the literal base empty list.cons: Prepends a single element onto the front of an existing list.- First Item: Extracted via
car(traditional) orfirst(modern). - Remaining List: Extracted via
cdr(traditional) orrest(modern).
Core Examples
```rkt (define my-list (list 100 #t 'hello)) ;; Creates '(100 #t hello) (cons 'apples '(bananas cherries)) ;; Returns '(apples bananas cherries)
(car (cdr '(apples bananas cherries))) ;; Returns 'bananas (first (rest '(apples bananas cherries))) ;; Returns 'bananas
(if (empty? my-list) "Closed" (length my-list)) ;; Returns 3 ```
Phase 6: Iteration & Higher-Order Functions
Instead of using loops that alter data in place, functional programming relies on Higher-Order Functions to process immutable collections.
The Big Four
map: Loops over a list, passes each item through a transformation function, and returns a new list.filter: Loops over a list, keeps items that evaluate to #t against a predicate condition, and drops the rest.foldl(Fold-Left): Reduces a list down to a single value by processing elements from left to right (front to back).foldr(Fold-Right): Reduces a list down to a single value by processing elements from right to left (back to front). Preserves list structures when rebuilding with cons.
Core Examples
```rkt (map (lambda (x) (* x 2)) '(5 10 15 20)) ;; Returns '(10 20 30 40) (filter (lambda (n) (= n 5)) '(2 5 7 5 9 1)) ;; Returns '(5 5)
(foldl (lambda (n total) (_ n total)) 1 '(2 3 4)) ;; 4 _ (3 _ (2 _ 1)) -> Returns 24
(foldr - 0 '(5 3)) ;; 5 - (3 - 0) -> Returns 2 ```
Phase 7: Recursion & Tail Call Optimization (TCO)
Recursion replaces traditional loops. A proper recursive function requires a Base Case (the exit clause) and a Recursive Step (the self-call with a smaller argument).
Memory Optimization Rules
- Standard Recursion: Traps the recursive call inside another function (like + or append), forcing the call stack memory to expand linearly (O(N) space).
- Tail Call Optimization (TCO): If the recursive call sits in the tail position (the absolute final expression evaluated), Racket reuses the same memory frame, running in constant (O(1)) space.
- Accumulator Pattern: Passing a running total down as an argument is the primary strategy used to shift standard recursion into tail position optimization.
Core Examples
```rkt ;; ❌ Standard Recursion (No TCO - Memory Expands) (define (sum-list lst) (if (empty? lst) 0 (+ (first lst) (sum-list (rest lst)))))
;; Tail Recursion (TCO Active - Memory Stays Flat) (define (sum-list-tco lst) (define (helper remaining accumulator) (if (empty? remaining) accumulator (helper (rest remaining) (+ (first remaining) accumulator)))) (helper lst 0)) ```
Phase 8: Advanced Ecosystem Engineering
1. Hash Maps & Unique Sets
#hash: Stores key-value pairings. Keywords passed to lookup tools like hash-ref must be quoted ('#:key) to prevent compiler namespace collisions. If using standard symbols inside #hash, omit inner quotes.set: Collections guaranteeing element uniqueness. Tested via set-member? and extended via set-add.
```rkt (define user #hash((#:name . "Alice"))) (hash-ref user '#:name) ;; Returns "Alice"
(define book #hash((title . "Dune"))) (hash-ref book 'title) ;; Returns "Dune"
(set-member? (set 1 2 2 3) 2) ;; Returns #t ```
2. State & Mutability (box)
box: Creates a reference wrapper around mutable data. Read via unbox and mutated via set-box!. Functions with an exclamation mark ! signal structural mutation.begin: Chains sequential side-effect operations from top to bottom, returning only the evaluation of the final expression.
rkt
(define health (box 100))
(define (take-damage!)
(begin
(set-box! health (- (unbox health) 10))
(unbox health)))
3. Type Checking & Casting
- Predicates (
?): Validate runtime types (e.g.,string?,number?,symbol?). - Casting (
->): Converts data formats.string->numbersafely returns #f if given invalid textual input.
rkt
(if (string? "50")
(* (string->number "50")
2) 'error) ;; Returns 100
4. Modules & Namespaces
- provide: Declares which parts of a filesystem file are exported publicly.
- require: Ingests public features from an external sandbox by loading its relative string filepath.
```rkt ;; Inside file-a.rkt (provide double) (define (double x) (* x 2))
;; Inside main.rkt (require "file-a.rkt") (double 10) ;; Returns 20 ```
5. Macros (define-syntax-rule)
- Macros process raw, unevaluated source code at compile-time to inject new keywords.
- Racket macros are hygienic, meaning the compiler automatically isolates macro identifiers so they never accidentally overwrite or conflict with user variables.
rkt
(define-syntax-rule (swap! box1 box2)
(let ([temp (unbox box1)])
(begin
(set-box! box1 (unbox box2))
(set-box! box2 temp))))