breezewiki/lib/syntax.rkt

#lang racket/base
(require (for-syntax racket/base))

(provide
 ; help make a nested if. if/in will gain the same false form of its containing if/out.
 if/out
 ; cond, but values can be defined between conditions
 cond/var
 ; wrap sql statements into lambdas so they can be executed during migration
 wrap-sql)

(module+ test
  (require rackunit)
  (define (check-syntax-equal? s1 s2)
    (check-equal? (syntax->datum s1)
                  (syntax->datum s2))))

;; actual transforming goes on in here.
;; it's in a submodule so that it can be required in both levels, for testing

(module transform racket/base
  (require racket/list)

  (provide
   transform-if/out
   transform/out-cond/var)

  (define (transform-if/out stx)
    (define tree (cdr (syntax->datum stx))) ; condition true false
    (define else (cddr tree)) ; the else branch cons cell
    (define result
      (let walk ([node tree])
        (cond
          ; normally, node should be a full cons cell (a pair) but it might be something else.
          ; situation: reached the end of a list, empty cons cell
          [(null? node) node]
          ; situation: reached the end of a list, cons cdr was non-list
          [(symbol? node) node]
          ; normal situation, full cons cell
          ; -- don't go replacing through nested if/out
          [(and (pair? node) (eq? 'if/out (car node))) node]
          ; -- replace if/in
          [(and (pair? node) (eq? 'if/in (car node)))
           (append '(if) (walk (cdr node)) else)]
          ; recurse down pair head and tail
          [(pair? node) (cons (walk (car node)) (walk (cdr node)))]
          ; something else that can't be recursed into, so pass it through
          [#t node])))
    (datum->syntax stx (cons 'if result)))

  (define (transform/out-cond/var stx)
    (define tree (transform-cond/var (cdr (syntax->datum stx))))
    (datum->syntax
     stx
     tree))

  (define (transform-cond/var tree)
    (define-values (els temp) (splitf-at tree (λ (el) (and (pair? el) (not (eq? (car el) 'var))))))
    (define-values (vars rest) (splitf-at temp (λ (el) (and (pair? el) (eq? (car el) 'var)))))
    (if (null? rest)
        `(cond ,@els)
        `(cond
           ,@els
           [#t
            (let* ,(for/list ([var vars])
                     (cdr var))
              ,(transform-cond/var rest))]))))

;; the syntax definitions and their tests go below here

(require 'transform (for-syntax 'transform))

(define-syntax (wrap-sql stx)
  ; the arguments
  (define xs (cdr (syntax->list stx)))
  ; wrap each argument
  (define wrapped (map (λ (xe) ; xe is the syntax of an argument
                         (if (list? (car (syntax->datum xe)))
                             ; it's a list of lists (a list of sql migration steps)
                             ; return instead syntax of a lambda that will call everything in xe
                             (datum->syntax stx `(λ () ,@xe))
                             ; it's just a single sql migration step
                             ; return instead syntax of a lambda that will call xe
                             (datum->syntax stx `(λ () ,xe))))
                       xs))
  ; since I'm returning *code*, I need to return the form (list ...) so that runtime makes a list
  (datum->syntax stx `(list ,@wrapped)))

(define-syntax (if/out stx)
  (transform-if/out stx))
(module+ test
  (check-syntax-equal? (transform-if/out #'(if/out (condition 1) (if/in (condition 2) (do-yes)) (do-no)))
                       #'(if (condition 1) (if (condition 2) (do-yes) (do-no)) (do-no)))
  (check-equal? (if/out #t (if/in #t 'yes) 'no) 'yes)
  (check-equal? (if/out #f (if/in #t 'yes) 'no) 'no)
  (check-equal? (if/out #t (if/in #f 'yes) 'no) 'no)
  (check-equal? (if/out #f (if/in #f 'yes) 'no) 'no))

(define-syntax (cond/var stx)
  (transform/out-cond/var stx))
(module+ test
  (check-syntax-equal? (transform/out-cond/var #'(cond/def [#f 0] (var d (* a 2)) [(eq? d 8) d] [#t "not 4"]))
                       #'(cond
                           [#f 0]
                           [#t
                            (let ([d (* a 2)])
                              (cond
                                [(eq? d 8) d]
                                [#t "not 4"]))])))