dsl - 如何在 Rebol 中使用 PARSE 评估 DSL？

Question

当我学习一些 DSL 时，我意识到 Rebol 中的 Parse 方言可以是一个很好的词法分析器和解析器。 the Parse tutorial 有一个很好的例子:

    expr:    [term ["+" | "-"] expr | term]
    term:    [factor ["*" | "/"] term | factor]
    factor:  [primary "**" factor | primary]
    primary: [some digit | "(" expr ")"]
    digit:   charset "0123456789"

    probe parse "4/5+3**2-(5*6+1)" expr
    ;will output true

上面的代码验证表达式是否符合上面定义的“语法”。我的问题是:

1. 如何计算或评估它？
2. 如何表示“*”、“+”等运算符的先验？

Answer 1

Boyko Bantchev 的 code 的修改版本:

arith-eval: funct [
    a-exp [string!]
    ] [
    op-stack: copy []  
    num-stack: copy []
    pop: func [stk /local t] [t: last stk   remove back tail stk t]
    do.op: func [/local op x y] [
        op: to-word pop op-stack  
        y: pop num-stack  
        x: pop num-stack
        append num-stack do reduce [x op y]
    ]
    num: op: none
    expr: [term any [copy op [{+} | {-}] (append op-stack op) term (do.op)]]
    term: [prim any [copy op [{*} | {/}] (append op-stack op) prim (do.op)]]
    prim: [copy num some digit (append num-stack to-decimal num) | {(} expr {)}]
    digit: charset {0123456789}
    ; whitespace is not allowed between tokens

    either parse a-exp expr  [num-stack/1] [{wrong expression}]
]



>> arith-eval "12+34*56-89/2"
== 1871.5
>>