combining.rkt

#lang racket/base

(require racket/match)

(define (combining xs pred?)
  (let loop ([xs xs])
    (match xs
      [(list* (? pred? this) (? pred? next) more)
       (loop (cons (string-append this next) more))]
      [(cons this more)
       (cons this (loop more))]
      [(list)
       (list)])))

(define (starts-with-A? s)
  (regexp-match? #px"^A" s))

(module+ test
  (require rackunit)
  (check-equal? (combining '("Apple" "Bear" "Alvin" "Anxious" "Agile" "Body")
                           starts-with-A?)
                '("Apple" "Bear" "AlvinAnxiousAgile" "Body")))