Beau­tiful Racket / racket school 2019

• 1 intro
• 2 pl check­list
• 3 lop till you drop
• 4 lop exam­ples
• 5 bullet train
• 6 trigram
• 7 flyover country
• 8 read only
• 9 expand only
• 10 conjunc­tion
• 11 injunc­tion

1. 

   Some more inter­po­si­tion points.

2. 

   #%datum: wraps literal data, so these:

   "str"
   42

   1 2

   "str" 42

   
   

   Become these:

   (#%datum . "str")
   (#%datum . 42)

   1 2	(#%datum . "str") (#%datum . 42)

   
   

   Ques­tion: what’s the meaning of the dot after #%datum?

   

   Ques­tion: why is the code below a bad idea? How would you fix it?

   (define-macro (#%datum . X)
     #'(list X X))
   42

   1 2 3

   (define-macro (#%datum . X) #'(list X X)) 42

   
3. 

   #%app: wraps func­tion appli­ca­tions, so this:

   (+ 42 -15)

   1	(+ 42 -15)

   
   

   Becomes this:

   (#%app + 42 -15)

   1	(#%app + 42 -15)

   
   

   Ques­tion: why is the code below a bad idea? How would you fix it?

   (define-macro (#%app OP ARG ...)
     #'(OP (abs ARG) ...))
   (+ 42 -15)

   1 2 3

   (define-macro (#%app OP ARG ...) #'(OP (abs ARG) ...)) (+ 42 -15)

   
4. 

   #%top: wraps iden­ti­fiers with top-level bind­ings, so id becomes (#%top . id). It can be used to handle unbound iden­ti­fiers.

   

   Ques­tion (hard): why is this so?

5. 

   Some more macrology.

6. 

   define-macro-cases lets you make a macro with several possible patterns, like cond:

   (define-macro-cases m
     [(m ARG1) #''one]
     [(m ARG1 ARG2) #''two]
     [(m . ARGS) #`#,(length (syntax->list #'ARGS))])
   
   (m foo) ; 'one
   (m foo 42) ; 'two
   (m foo 42 "yeah") ; 3

   1 2 3 4 5 6 7 8

   (define-macro-cases m [(m ARG1) #''one] [(m ARG1 ARG2) #''two] [(m . ARGS) #`#,(length (syntax->list #'ARGS))]) (m foo) ; 'one (m foo 42) ; 'two (m foo 42 "yeah") ; 3

   
   

   Ques­tion: what’s the meaning of #' #'' and #`#,?

7. 

   syntax->datum lets you peek inside a syntax object to see what’s inside. Use this with care, because you’re working at compile time, not run time.

   (define-macro (m THING)
     (define datum (syntax->datum #'THING))
     (cond
       [(number? datum) #'"wow!"]
       [else #'THING]))
   
   (m 99)
   (m "foo")
   (define x 42)
   (m x)

   1 2 3 4 5 6 7 8 9 10

   (define-macro (m THING) (define datum (syntax->datum #'THING)) (cond [(number? datum) #'"wow!"] [else #'THING])) (m 99) (m "foo") (define x 42) (m x)

   
   

   Ques­tion: why is the last result 42, not "wow"? Isn’t x holding a number?

injunction/test.rkt

#lang injunction

"hello world"
(+ 1 (* 2 (- x)))

1 2 3 4

#lang injunction "hello world" (+ 1 (* 2 (- x)))



"whee"
'(kaboom 42 (kaboom 42 (kaboom kaboom)))

1 2	"whee" '(kaboom 42 (kaboom 42 (kaboom kaboom)))

injunction/main.rkt

#lang br

;; smop ···

(module reader syntax/module-reader
  injunction)

1 2 3 4 5 6

#lang br ;; smop ··· (module reader syntax/module-reader injunction)

• 1 intro
• 2 pl check­list
• 3 lop till you drop
• 4 lop exam­ples
• 5 bullet train
• 6 trigram
• 7 flyover country
• 8 read only
• 9 expand only
• 10 conjunc­tion
• 11 injunc­tion