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Beau­tiful Racket / racket school 2019

Day 2

Powerful points

Wherein the grammar is revealed to be even more valu­able.

  1. Recall that the expander supplies the initial bind­ings for the program.

  2. Recall that a brag parser produces a parse tree where each node starts with the name of a produc­tion rule, followed by the matched elements.

  3. The expander coop­er­ates with the parser by exporting bind­ings that corre­spond to those produc­tion rules.

  4. In this way, the parse tree becomes part of our program. For instance, this expres­sion:

    (with-syntax ([PT parse-tree])
      #'(module winner taco-victory
          (display (apply string PT)))
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    (with-syntax ([PT parse-tree])
  #'(module winner taco-victory
      (display (apply string PT)))
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    Might receive a parse-tree like this:

    #'(taco-program (taco-leaf (not-a-taco) (taco)))
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    #'(taco-program (taco-leaf (not-a-taco) (taco)))
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    And return this composite syntax object (where the pattern vari­able PT in the syntax template is replaced with the value of parse-tree):

    #'(module winner taco-victory
        (display (apply string (taco-program (taco-leaf (not-a-taco) (taco))))))
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    #'(module winner taco-victory
    (display (apply string (taco-program (taco-leaf (not-a-taco) (taco))))))
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  5. The name of the produc­tion rule becomes the iden­ti­fier of a new func­tion (or macro) provided by the expander.

  6. For each leaf of the parse tree, the matched elements of the produc­tion rule become the input to that iden­ti­fier.

  7. Thus, the grammar not only gener­ates our parser, but gives us a spec­i­fi­ca­tion for the expander.

  8. The expander needs to export bind­ings for every iden­ti­fier that appears in the parse tree. These bind­ings can be macros or ordi­nary func­tions.

Mission

Make a language called taco-victory that takes the same input as tacopocalypse and converts it back into a Racket program. Use what­ever combi­na­tion of lexer, parser, and expander makes life easiest.

Example:

taco-victory/test.rkt
#lang taco-victory
##$%#$%#$#$#$$##$%#$%#$#$#$$##$%#$#$#$%#$$##$#$#$%#$%%$#%#$%#$#$%%$##$#$%%#$%%$##$#$%%#$%%$#%%%%#$%%$##$#$#$#$#$%#$$#%%%#$%%%$#%%%%#$%%$##$%#$#$%%%$##$#$%%#$%%$##$#$%#$#$%%$##$%#$#$#$%#$$##$%#$%#$#$#$$##$#$#$%#$%#$$#%%#$%#$%#$$##$#$#$#$#$%#$$#%#$#$#$%%#$$##$#$#$#$#$%#$$##$#$#$%#$%#$$##$%#$%#$%#$$##$#$#$#$#$%#$$##$%#$#$%%#$$##$#$#$#$#$%#$$##$#$#$%#$%#$$#%#$%%#$%#$$##$#$#$#$#$%#$$##$#$#$%%%%$#%#$#$%#$%#$$#%#$#$%#$%#$$#%#$#$%#$%#$$
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#lang taco-victory
##$%#$%#$#$#$$##$%#$%#$#$#$$##$%#$#$#$%#$$##$#$#$%#$%%$#%#$%#$#$%%$##$#$%%#$%%$##$#$%%#$%%$#%%%%#$%%$##$#$#$#$#$%#$$#%%%#$%%%$#%%%%#$%%$##$%#$#$%%%$##$#$%%#$%%$##$#$%#$#$%%$##$%#$#$#$%#$$##$%#$%#$#$#$$##$#$#$%#$%#$$#%%#$%#$%#$$##$#$#$#$#$%#$$#%#$#$#$%%#$$##$#$#$#$#$%#$$##$#$#$%#$%#$$##$%#$%#$%#$$##$#$#$#$#$%#$$##$%#$#$%%#$$##$#$#$#$#$%#$$##$#$#$%#$%#$$#%#$%%#$%#$$##$#$#$#$#$%#$$##$#$#$%%%%$#%#$#$%#$%#$$#%#$#$%#$%#$$#%#$#$%#$%#$$
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Result:

 
 
"hello world"
(+ 1 (* 2 (- x)))
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"hello world"
(+ 1 (* 2 (- x)))
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Hint: You need four rules in your grammar, and four corre­sponding func­tions in your expander. Code to get you started is below. Note one wrinkle: this time, parse calls tokenize repeat­edly to get each token (this approach was mentioned in quantum-taco). So this tokenize should return one token at a time, rather than all of them.

taco-victory/main.rkt
#lang br/quicklang
(require brag/support "grammar.rkt")
(provide taco-program taco-leaf
         taco not-a-taco
         show
         #%module-begin)

(module+ reader
  (provide read-syntax))

(define (tokenize-1 ip)
  ;; return one token (not all tokens)
  ;; ···
  )

(define (taco-program . pieces)
  ;; ···
  )

(define (taco-leaf . pieces)
  ;; ···
  )

(define (taco)
  ;; ···
  )

(define (not-a-taco)
  ;; ···
  )

(define (show pt)
  (display (apply string pt)))

(define (read-syntax src ip)
  (define token-thunk (λ () (tokenize-1 ip)))
  (define parse-tree (parse src token-thunk))
  (strip-bindings
   (with-syntax ([PT parse-tree])
     #'(module winner taco-victory
         (show PT)))))
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#lang br/quicklang
(require brag/support "grammar.rkt")
(provide taco-program taco-leaf
         taco not-a-taco
         show
         #%module-begin)

(module+ reader
  (provide read-syntax))

(define (tokenize-1 ip)
  ;; return one token (not all tokens)
  ;; ···
  )

(define (taco-program . pieces)
  ;; ···
  )

(define (taco-leaf . pieces)
  ;; ···
  )

(define (taco)
  ;; ···
  )

(define (not-a-taco)
  ;; ···
  )

(define (show pt)
  (display (apply string pt)))

(define (read-syntax src ip)
  (define token-thunk (λ () (tokenize-1 ip)))
  (define parse-tree (parse src token-thunk))
  (strip-bindings
   (with-syntax ([PT parse-tree])
     #'(module winner taco-victory
         (show PT)))))
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taco-victory/grammar.rkt
#lang brag
taco-program : ;; ···
taco-leaf : ;; ···
taco : ;; ···
not-a-taco : ;; ···
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#lang brag
taco-program : ;; ···
taco-leaf : ;; ···
taco : ;; ···
not-a-taco : ;; ···
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Hint: (apply string PT) will only succeed if the value repre­sented by PT eval­u­ates to a list of char­ac­ters.

Hint: work on "main.rkt" in isola­tion until this REPL input:

(apply-port-proc tokenize-1 "\n#$#$#$%#$%%\n")
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(apply-port-proc tokenize-1 "\n#$#$#$%#$%%\n")
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Produces:

'("#$" "#$" "#$" "%" "#$" "%" "%")
1
'("#$" "#$" "#$" "%" "#$" "%" "%")
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Hint: work on "grammar.rkt" in isola­tion until this REPL input:

(parse-to-datum '("#$" "#$" "#$" "%" "#$" "%" "%"))
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(parse-to-datum '("#$" "#$" "#$" "%" "#$" "%" "%"))
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Produces:

'(taco-program (taco-leaf (not-a-taco) (not-a-taco) (not-a-taco) (taco) (not-a-taco) (taco) (taco)))
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'(taco-program (taco-leaf (not-a-taco) (not-a-taco) (not-a-taco) (taco) (not-a-taco) (taco) (taco)))
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Hint: work on "main.rkt" in isola­tion until this REPL input:

(taco-program (taco-leaf (not-a-taco) (not-a-taco) (not-a-taco) (taco) (not-a-taco) (taco) (taco)))
1
(taco-program (taco-leaf (not-a-taco) (not-a-taco) (not-a-taco) (taco) (not-a-taco) (taco) (taco)))
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Produces:

'(#\h)
1
'(#\h)
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Summary

It’s all down­hill from here. For some defi­n­i­tion of “all”.

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