CODEWITHSUNDEEP
macrosschemeread-eval-print-loopr5rs
Georg Summer
Georg Summer

Reputation: 13

How to map a macro over a list - or - How to use macros to define data types

I like to build a REPL with my own datatypes, but I don't like to write all the same pattern functions over and over again.

So this is a nut, which bothers me.

I got my own set of primitive datatypes (define primitives '("mytrue" "myfalse" "mynumber" ...))

Also I have (define primitiveTesters (list "mytrue?" "myfalse?" "mynumber?" ... )

The problem now is, I just want to apply (map) or a macro to get the datatype? procedurces, which basically just checks if the car of record (mynumber . ( . )) exists.

So something similar like (mynumber? (car (mynumber.(1.))) => #t in the end. But for this I need (define mynumber? (lambda (...)(...))

My define-batching macro looks like this, but I just have no luck to infuse the <variable>. 

(define-syntax define-batching
(syntax-rules () 
((_ value expr)(define value expr))
((_ value) value)
((_ value1 value2 ...)  (begin (define value1 expr) (define-batching test2...)))
))

So have I reached a dead end of scheme ?

I've seen something similar, I think in Emacs Lisp.

What I am looking for in the end is: 

 (define checker '(audi? volkswagen? mercedes?))
 (define datatype '(audi volkswagen mercedes))

 (map define-checker checker datatype )
 or
 (define-checker (car checker) (car datatype))

Upvotes: 1

Views: 337

Answers (2)

soegaard
soegaard

Reputation: 31145

If I understood the question right, you need a macro to define your own type checkers?

Here is one way to do it:

(define-syntax define-checker
  (syntax-rules ()
    [(define-checker name tag)
     (define (name object)
       (and (list? object)
            (not (null? object))
            (eq? (car object) 'tag)))]))

(define-checker my-car? car)

(my-car? '(car audi black))   ; evaluates to #t
(my-car? '(truck ford pink))  ; evaluates to #f

Addendum:

If you write

(define checker '(audi? volkswagen? mercedes?))
(define datatype '(audi volkswagen mercedes))

the values will become available at runtime. Therefore you need to a different approach.

You could for example write:

(define-checker+datatype (audi? audi) (volkswagen? volkswagen?))

Here is the code:

(define-syntax define-checker
  (syntax-rules ()
    [(define-checker name tag)
     (define (name object)
       (and (list? object)
            (not (null? object))
            (eq? (car object) 'tag)))]))


(define-syntax define-checkers+datatype
  (syntax-rules ()
    [(define-checkers+datatype (name tag) ...)
     (begin
       (define-checker name tag)
       ...)]))

(define-checkers+datatype (audi? audi) (wv? wv))


(audi? '(audi black))

Upvotes: 2

paul
paul

Reputation: 1150

define-syntax is hygienic, that means it cannot influence on parent environment, that means it cannot define symbols in it. You may try to use er-, ir- macro-transformers which allow you to explicit renames symbols.

keywords to google in you scheme documentation are 'er-macro-transformet' and 'ir-macro-transformer'

Upvotes: 0

Related Questions