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chore: DRY out expression parser #9

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merged 1 commit into from
Jan 11, 2024
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chore: DRY out expression parser #9

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66 changes: 21 additions & 45 deletions src/parser.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -82,51 +82,6 @@ export function parse(tokens: Token[]) {
// expression → equality ;
const expression = () => equality();

// equality → comparison ( ( "!=" | "==" ) comparison )* ;
const equality = (): Expr => {
let expr: Expr = comparison();
while (match("!=", "==")) {
const operator = previous();
const right = comparison();
expr = { kind: "binary", left: expr, operator, right };
}
return expr;
};

// TODO: this is very similar to equality()
// comparison → term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
const comparison = (): Expr => {
let expr: Expr = term();
while (match(">", ">=", "<", "<=")) {
const operator = previous();
const right = term();
expr = { kind: "binary", left: expr, operator, right };
}
return expr;
};

// term → factor ( ( "-" | "+" ) factor )* ;
const term = (): Expr => {
let expr: Expr = factor();
while (match("-", "+")) {
const operator = previous();
const right = factor();
expr = { kind: "binary", left: expr, operator, right };
}
return expr;
};

// factor → unary ( ( "/" | "*" ) unary )* ;
const factor = (): Expr => {
let expr: Expr = unary();
while (match("/", "*")) {
const operator = previous();
const right = unary();
expr = { kind: "binary", left: expr, operator, right };
}
return expr;
};

// unary → ( "!" | "-" ) unary | primary ;
const unary = (): Expr => {
if (match("!", "-")) {
Expand Down Expand Up @@ -154,6 +109,27 @@ export function parse(tokens: Token[]) {
}
throw error(peek(), "Expect expression.");
};

// Creates a parsing function for a rule of this form:
// rule → next ( ( any of ops ) next )* ;
const parseBinaryOp = (ops: TokenType[], next: () => Expr) => {
return (): Expr => {
let expr: Expr = next();
while (match(...ops)) {
const operator = previous();
const right = next();
expr = { kind: "binary", left: expr, operator, right };
}
return expr;
};
};

// These productions are all of the form:
// equality → comparison ( ( "!=" | "==" ) comparison )* ;
const factor = parseBinaryOp(["/", "*"], unary);
const term = parseBinaryOp(["-", "+"], factor);
const comparison = parseBinaryOp([">", ">=", "<", "<="], term);
const equality = parseBinaryOp(["!=", "=="], comparison);
// #endregion

try {
Expand Down
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