74 lines
No EOL
1.6 KiB
JavaScript
74 lines
No EOL
1.6 KiB
JavaScript
import { fixer } from "../lib/util/fixpoint.js";
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const booleanLattice = {
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isMaximal: (value) => value,
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equality: (l, r) => l === r,
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bottom: () => false,
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};
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const nullable = grammar => nonterminal => request => {
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console.log(nonterminal);
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const f = nt => {
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return {
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Epsilon: () => true,
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T: () => false,
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NT: () => request(nt.nt),
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Seq: () => f(nt.l) && f(nt.r),
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Alt: () => f(nt.l) || f(nt.r),
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}[nt.kind]();
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}
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return f(grammar[nonterminal]);
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}
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// S → A B C
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// A → a A | ε
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// B → b B | A C
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// C → c C | ε
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const exampleGrammar = {
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S: {kind: "Seq",
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l: {kind: "NT", nt: "A"},
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r: {kind: "Seq",
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l: {kind: "NT", nt: "B"},
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r: {kind: "NT", nt: "C"},
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},
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},
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A: {kind: "Alt",
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l: {kind: "Seq",
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l: {kind: "T", token: "a"},
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r: {kind: "NT", nt: "A"}
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},
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r: {kind: "Epsilon"},
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},
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B: {kind: "Alt",
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l: {kind: "Seq",
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l: {kind: "T", token: "b"},
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r: {kind: "NT", nt: "B"}
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},
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r: {kind: "Seq",
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l: {kind: "NT", nt: "A"},
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r: {kind: "NT", nt: "C"}
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},
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},
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C: {kind: "Alt",
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l: {kind: "Seq",
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l: {kind: "T", token: "c"},
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r: {kind: "NT", nt: "C"}
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},
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r: {kind: "Epsilon"},
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},
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};
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// A is nullable because it has a production A → ε.
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// C is nullable because it has a production C → ε.
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// B is nullable because B → A C, and both A and C are nullable.
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// S is nullable because S → A B C, and A, B, and C are all nullable.
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const isNullable = fixer(nullable(exampleGrammar), booleanLattice);
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console.log(isNullable("S"));
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console.log(isNullable("A"));
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console.log(isNullable("B"));
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console.log(isNullable("C")); |