Unhygienic macros inside of unhygienic macros are difficult

In this post; Ryan explains why unhygienic macros inside of unhygienic macros are often difficult.

;; the working if-it & when-it
 
(define-syntax (if-it stx)
  (syntax-case stx ()
    ((if-it test? then else)
     (with-syntax ((it (datum->syntax #'if-it 'it)))
       #'(let ((it test?))
           (if it then else)))))) 
 
(define-syntax (when-it stx)
  (syntax-case stx ()
    ((~ test? exp exp2 ...)
     (with-syntax ((it (datum->syntax #'~ 'it)))
       #'(let ((it test?)) (when it exp exp2 ...))))))
 
;; the non-working cond-it 
(define-syntax (cond-it stx)
  (syntax-case stx (else)
    ((cond-it (else exp exp2 ...))
     #'(begin exp exp2 ...))
    ((cond-it (test? exp exp2 ...))
     #'(when-it test? exp exp2 ...))
    ((cond-it (test? exp exp2 ...) cond1 cond2 ...)
     #'(if-it test? (begin exp exp2 ...)
              (cond-it cond1 cond2 ...)))))

When ‘cond-it’ expands and produces an ‘if-it’ expression, the ‘if-it’ is marked by the macro expander as coming from a macro. That means its lexical context is different from the ‘it’ variables in the branches. That means that the ‘it’ variable binding produced by ‘if-it’ does not capture the ‘it’ references in the branches.

— Ryan

Wisdom and Wonder

Unhygienic macros inside of unhygienic macros are difficult

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