Go with the flow: basic

#lang basic
30 rem print 'ignored'
35
50 print "never gets here"
40 end
60 print 'three' : print 1.0 + 3
70 goto 11. + 18.5 + .5 rem ignored
10 print "o" ; "n" ; "e"
20 print : goto 60.0 : end

#lang br/quick­lang
(require "parser.rkt" "tokenizer.rkt")

(define (read-syntax path port)
  (define parse-tree (parse path (make-tokenizer port path)))
  (strip-bind­ings
   #`(module basic-mod basic/expander
       #,parse-tree)))

(module+ reader
  (provide read-syntax))

#lang br
(require brag/support)

(define-lex-abbrev digits (:+ (char-set "0123456789")))

(define basic-lexer
  (lexer-srcloc
   ["\n" (token 'NEWLINE lexeme)]
   [white­space (token lexeme #:skip? #t)]
   [(from/stop-before "rem" "\n") (token 'REM lexeme)]
   [(:or "print" "goto" "end"
         "+" ":" ";") (token lexeme lexeme)]
   [digits (token 'INTEGER (string->number lexeme))]
   [(:or (:seq (:? digits) "." digits)
         (:seq digits "."))
    (token 'DECIMAL (string->number lexeme))]
   [(:or (from/to "\"" "\"") (from/to "'" "'"))
    (token 'STRING
           (substring lexeme
                      1 (sub1 (string-length lexeme))))]))

(provide basic-lexer)

#lang br
(require "lexer.rkt" brag/support rack­unit)

(define (lex str)
  (apply-port-proc basic-lexer str))

(check-equal? (lex "") empty)
(check-equal?
 (lex " ")
 (list (srcloc-token (token " " #:skip? #t)
                     (srcloc 'string 1 0 1 1))))
(check-equal?
 (lex "rem ignored\n")
 (list (srcloc-token (token 'REM "rem ignored")
                     (srcloc 'string 1 0 1 11))
       (srcloc-token (token 'NEWLINE "\n")
                     (srcloc 'string 1 11 12 1))))
(check-equal?
 (lex "print")
 (list (srcloc-token (token "print" "print")
                     (srcloc 'string 1 0 1 5))))
(check-equal?
 (lex "goto")
 (list (srcloc-token (token "goto" "goto")
                     (srcloc 'string 1 0 1 4))))
(check-equal?
 (lex "end")
 (list (srcloc-token (token "end" "end")
                     (srcloc 'string 1 0 1 3))))
(check-equal?
 (lex "+")
 (list (srcloc-token (token "+" "+")
                     (srcloc 'string 1 0 1 1))))
(check-equal?
 (lex ";")
 (list (srcloc-token (token ";" ";")
                     (srcloc 'string 1 0 1 1))))
(check-equal?
 (lex ":")
 (list (srcloc-token (token ":" ":")
                     (srcloc 'string 1 0 1 1))))
(check-equal?
 (lex "12")
 (list (srcloc-token (token 'INTEGER 12)
                     (srcloc 'string 1 0 1 2))))
(check-equal?
 (lex "1.2")
 (list (srcloc-token (token 'DECIMAL 1.2)
                     (srcloc 'string 1 0 1 3))))
(check-equal?
 (lex "12.")
 (list (srcloc-token (token 'DECIMAL 12.)
                     (srcloc 'string 1 0 1 3))))
(check-equal?
 (lex ".12")
 (list (srcloc-token (token 'DECIMAL .12)
                     (srcloc 'string 1 0 1 3))))
(check-equal?
 (lex "\"foo\"")
 (list (srcloc-token (token 'STRING "foo")
                     (srcloc 'string 1 0 1 5))))
(check-equal?
 (lex "'foo'")
 (list (srcloc-token (token 'STRING "foo")
                     (srcloc 'string 1 0 1 5))))
(check-exn exn:fail:read? (lambda () (lex "x")))

#lang br
(require "lexer.rkt" brag/support)

(define (make-tokenizer ip [path #f])
  (port-count-lines! ip)
  (lexer-file-path path)
  (define (next-token) (basic-lexer ip))
  next-token)

(provide make-tokenizer)

#lang brag
b-program : [b-line] (/NEWLINE [b-line])*
b-line : b-line-num [b-state­ment] (/":" [b-state­ment])* [b-rem]
@b-line-num : INTEGER
@b-state­ment : b-end | b-print | b-goto
b-rem : REM
b-end : /"end"
b-print : /"print" [b-print­able] (/";" [b-print­able])*
@b-print­able : STRING | b-expr
b-goto : /"goto" b-expr
b-expr : b-sum
b-sum : b-number (/"+" b-number)*
@b-number : INTEGER | DECIMAL

#lang br/quick­lang
(require "parser.rkt" "tokenizer.rkt")

(define (read-syntax path port)
  (define parse-tree (parse path (make-tokenizer port path)))
  (strip-bind­ings
   #`(module basic-parser-mod basic/parse-only
       #,parse-tree)))
(module+ reader (provide read-syntax))

(define-macro (parser-only-mb PARSE-TREE)
  #'(#%module-begin
     'PARSE-TREE))
(provide (rename-out [parser-only-mb #%module-begin]))

#lang br/quick­lang
(require brag/support "tokenizer.rkt")

(define (read-syntax path port)
  (define tokens (apply-tokenizer make-tokenizer port))
  (strip-bind­ings
   #`(module basic-tokens-mod basic/tokenize-only
       #,@tokens)))
(module+ reader (provide read-syntax))

(define-macro (tokenize-only-mb TOKEN ...)
  #'(#%module-begin
     (list TOKEN ...)))
(provide (rename-out [tokenize-only-mb #%module-begin]))

#lang br/quick­lang
(provide (matching-iden­ti­fiers-out #rx"^b-" (all-defined-out)))

(define-macro (b-line NUM STATE­MENT ...)
  (with-pattern ([LINE-NUM (prefix-id "line-" #'NUM
                                      #:source #'NUM)])
    (syntax/loc caller-stx
      (define (LINE-NUM) (void) STATE­MENT ...))))

(define-macro (b-module-begin (b-program LINE ...))
  (with-pattern
      ([((b-line NUM STMT ...) ...) #'(LINE ...)]
       [(LINE-FUNC ...) (prefix-id "line-" #'(NUM ...))])
    #'(#%module-begin
       LINE ...
       (define line-table
         (apply hasheqv (append (list NUM LINE-FUNC) ...)))
       (void (run line-table)))))
(provide (rename-out [b-module-begin #%module-begin]))

(struct end-program-signal ())
(struct change-line-signal (val))

(define (b-end) (raise (end-program-signal)))
(define (b-goto expr) (raise (change-line-signal expr)))

(define (run line-table)
  (define line-vec
    (list->vector (sort (hash-keys line-table) <)))
  (with-handlers ([end-program-signal? (λ (exn-val) (void))])
    (for/fold ([line-idx 0])
              ([i (in-natu­rals)]
               #:break (>= line-idx (vector-length line-vec)))
      (define line-num (vector-ref line-vec line-idx))
      (define line-func (hash-ref line-table line-num))
      (with-handlers
          ([change-line-signal?
            (λ (cls)
              (define clsv (change-line-signal-val cls))
              (or
               (and (exact-posi­tive-integer? clsv)
                    (vector-member clsv line-vec))
               (error
                (format "error in line ~a: line ~a not found"
                        line-num clsv))))])
        (line-func)
        (add1 line-idx)))))

(define (b-rem val) (void))
(define (b-print . vals)
  (displayln (string-append* (map ~a vals))))
(define (b-sum . vals) (apply + vals))
(define (b-expr expr)
  (if (integer? expr) (inexact->exact expr) expr))