Consider the difference in results below. How would I persuade `syntax-parse`
to match literal identifiers in the `free-identifier=?` sense that
`syntax-case` does? (and thereby allow unbound identifiers to be compared also)
IIUC, `#:literals` is too strict, as it insists that every listed identifier
have a binding; `#:datum-literals` is too lenient, as it does not compare
bindings at all, but rather just the symbol.
;;;;;;;;;;;;;;;;;;;;;;;;;
#lang racket
(require math rackunit)
(module mod racket
(require (for-syntax syntax/parse))
(define-syntax (mac stx)
(syntax-parse stx
#:literals () ;; <- can't put `zeta` here, produces an error
#:datum-literals (zeta)
[(_ zeta) #''match]
[else #''nope]))
(provide mac))
(require 'mod)
(check-equal? (mac zeta) 'match)
(module mod2 racket
(require (for-syntax syntax/parse))
(define-syntax (mac2 stx)
(syntax-case stx (zeta)
[(_ zeta) #''match]
[else #''nope]))
(provide mac2))
(require 'mod2)
(check-equal? (mac2 zeta) 'nope)
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