My first Typed Racket program: the first pieces of a parser.

parser.rkt

#lang typed/racket

(provide (all-defined-out))

(define-type Stream (Listof Char))

(struct: Success ([match : Any] [rest : Stream]) #:transparent)
(struct: Fail () #:transparent)

(define-type Result (U Success Fail))

(define-type Parser (Stream -> Result))

(: any-char (Stream -> Result))
(define (any-char s)
  (if (null? s)
    (Fail)
    (Success (first s) (rest s))))

(: all-of (Parser * -> Parser))
(define (all-of . tests)
  (lambda (s)
    (: loop (Result (Listof Parser) -> Result))
    (define (loop c tests)
      (if (null? tests)
        c
        (match ((first tests) s)
           [(Success c cs) (loop (Success c cs) (rest tests))]
           [_ (Fail)])))
    (loop (Fail) tests)))