这里有一种方法。首先定义文本(如果在SELECT之外使用,则给出一条很好的错误消息),然后提供它们。
#lang racket
(provide SELECT FROM WHERE)
(define-syntax FROM (λ (stx) (raise-syntax-error 'FROM "literal FROM used outside SELECT" stx)))
(define-syntax WHERE (λ (stx) (raise-syntax-error 'WHERE "literal WHERE used outside SELECT" stx)))
(define-syntax SELECT
(syntax-rules (FROM WHERE star)
[(SELECT colnames FROM relnames) ...]
[(SELECT colnames FROM relnames WHERE . expression) ...]))