AdventOfCode-2023-functional/day01-easy/main.scm

#!/usr/bin/guile -s
!#

(use-modules (ice-9 rdelim))

(define (read-lines)
  (let loop ((line (read-line)))
    (if
      (eof-object? line)
      '()
      (cons line (loop (read-line))))))

(define (map list mapper)
  (if
    (null? list)
    '()
    (cons (mapper (car list)) (map (cdr list) mapper))))

(define (reduce list reducer current)
  (if
    (null? list)
    current
    (reduce (cdr list) reducer (reducer current (car list)))))

(define (combine a b combiner)
  (if
    (null? a)
    b
    (if
      (null? b)
      a
      (combiner a b))))

(define (sum numbers) (reduce numbers + 0))

(define
  (repeat value number)
  (if
    (= number 0)
    '()
    (cons value (repeat value (- number 1)))))

(define (is-numeric-char char) (if (char-numeric? char) #t #f))

(define (first-last list predicate)
  (reduce
    (map list (lambda (entry)
      (if (predicate entry) (repeat entry 2) '())))
    (lambda (current accumulator)
      (combine
        current
        accumulator
        (lambda (current rest)
          (cons (car current) (cdr rest)))))
    '()))

(define (solve-line line)
  ((lambda (first-last-result)
    (string->number (list->string first-last-result)))
    (first-last (string->list line) is-numeric-char)))

(define (solve-all lines)
  (sum (map lines solve-line)))

(display (solve-all (read-lines)))