e510ecc3d0
In the past year I worked on various grammars where first/rest or head/tail were used as labels for parts of lists. I found I associate head/tail with a list immediately, while in case of first/rest I have to "parse" grammar rules for a while before understanding their structure. Moreover, I tend to assume that rest is a list of the same thigs as first, but I don't have such assumption in case of head/tail. This assumption was in conflict with the grammar structure. I'm not sure how much these observations are applicable to others, but I decided to act on them and switch from first/rest to head/tail.
41 lines
859 B
JavaScript
41 lines
859 B
JavaScript
/*
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* Simple Arithmetics Grammar
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* ==========================
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*
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* Accepts expressions like "2 * (3 + 4)" and computes their value.
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*/
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Expression
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= head:Term tail:(_ ("+" / "-") _ Term)* {
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var result = head, i;
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for (i = 0; i < tail.length; i++) {
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if (tail[i][1] === "+") { result += tail[i][3]; }
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if (tail[i][1] === "-") { result -= tail[i][3]; }
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}
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return result;
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}
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Term
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= head:Factor tail:(_ ("*" / "/") _ Factor)* {
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var result = head, i;
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for (i = 0; i < tail.length; i++) {
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if (tail[i][1] === "*") { result *= tail[i][3]; }
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if (tail[i][1] === "/") { result /= tail[i][3]; }
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}
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return result;
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}
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Factor
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= "(" _ expr:Expression _ ")" { return expr; }
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/ Integer
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Integer "integer"
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= [0-9]+ { return parseInt(text(), 10); }
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_ "whitespace"
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= [ \t\n\r]*
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