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Interpreter Pattern

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Before: full revision: 2026-10-25 00:14:24 UTC After: full revision: 2026-10-25 03:10:53 UTC

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using System; using System.Collections.Generic; using System.Linq; IExpr rule = new HasTag("article"); Console.WriteLine(rule.Eval(new HashSet<string> { "article" })); interface IExpr { bool Eval(IReadOnlySet<string> tags); } sealed record HasTag(string Tag) : IExpr { public bool Eval(IReadOnlySet<string> tags) => tags.Contains(Tag); } sealed record And(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) && Right.Eval(tags); } sealed record Or(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) || Right.Eval(tags); }

using System; using System.Collections.Generic; using System.Linq; IExpr rule = new HasTag("article"); Console.WriteLine(rule.Eval(new HashSet<string> { "article" })); // Different expression kinds evaluate the same input to a boolean result. interface IExpr { bool Eval(IReadOnlySet<string> tags); } sealed record HasTag(string Tag) : IExpr { public bool Eval(IReadOnlySet<string> tags) => tags.Contains(Tag); } // Delegate to child expressions and own only the combination rule. sealed record And(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) && Right.Eval(tags); } sealed record Or(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) || Right.Eval(tags); }

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using System; using System.Collections.Generic; using System.Linq; IExpr rule = new Or(new HasTag("article"), new And(new HasTag("video"), new HasTag("captioned"))); foreach (var tags in new[] { new HashSet<string> { "article" }, new HashSet<string> { "video" }, new HashSet<string> { "video", "captioned" } }) Console.WriteLine(rule.Eval(tags)); interface IExpr { bool Eval(IReadOnlySet<string> tags); } sealed record HasTag(string Tag) : IExpr { public bool Eval(IReadOnlySet<string> tags) => tags.Contains(Tag); } sealed record And(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) && Right.Eval(tags); } sealed record Or(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) || Right.Eval(tags); }

using System; using System.Collections.Generic; using System.Linq; IExpr rule = new Or(new HasTag("article"), new And(new HasTag("video"), new HasTag("captioned"))); foreach (var tags in new[] { new HashSet<string> { "article" }, new HashSet<string> { "video" }, new HashSet<string> { "video", "captioned" } }) Console.WriteLine(rule.Eval(tags)); // Different expression kinds evaluate the same input to a boolean result. interface IExpr { bool Eval(IReadOnlySet<string> tags); } sealed record HasTag(string Tag) : IExpr { public bool Eval(IReadOnlySet<string> tags) => tags.Contains(Tag); } // Delegate to child expressions and own only the combination rule. sealed record And(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) && Right.Eval(tags); } sealed record Or(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) || Right.Eval(tags); }

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TypeScript example Expression objects separate a single tag test from AND and OR combinations. Callers compose an expression tree and evaluate it through eval() instead of rebuilding a large conditional. // Different expression kinds evaluate the same input to a boolean result. interface IExpr { eval(tags: ReadonlySet<string>): boolean; } class HasTag implements IExpr { constructor(private readonly tag: string) {} eval(tags: ReadonlySet<string>): boolean { return tags.has(this.tag); } } // Delegate to child expressions and own only the combination rule. class And implements IExpr { constructor(private readonly left: IExpr, private readonly right: IExpr) {} eval(tags: ReadonlySet<string>): boolean { return this.left.eval(tags) && this.right.eval(tags); } } class Or implements IExpr { constructor(private readonly left: IExpr, private readonly right: IExpr) {} eval(tags: ReadonlySet<string>): boolean { return this.left.eval(tags) || this.right.eval(tags); } } const rule: IExpr = new Or(new HasTag("article"), new And(new HasTag("video"), new HasTag("captioned"))); for (const tags of [["article"], ["video", "captioned"], ["video"]]) { console.log(rule.eval(new Set(tags))); } true true false AND and OR evaluate their right operand only when needed. The example builds expressions directly as objects; it is not a string parser or an arbitrary-code evaluator.

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Each example is a separate .NET 10 console program.

Each C# example is a separate .NET 10 console program. Compile the TypeScript examples for ES2022 or later and run the emitted JavaScript with Node.js.

Interpreter Pattern

Body changes between the two selected stored revisions. Code is not executed.

5 changed sections

− Removed+ Added20 unchanged content blocks omitted.

Content blocks 5 → 5

The text is unchanged, but footnotes, formatting or block attributes have changed.

Content blocks 14 → 14

using System; using System.Collections.Generic; using System.Linq; IExpr rule = new HasTag("article"); Console.WriteLine(rule.Eval(new HashSet<string> { "article" })); // Different expression kinds evaluate the same input to a boolean result. interface IExpr { bool Eval(IReadOnlySet<string> tags); } sealed record HasTag(string Tag) : IExpr { public bool Eval(IReadOnlySet<string> tags) => tags.Contains(Tag); } // Delegate to child expressions and own only the combination rule. sealed record And(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) && Right.Eval(tags); } sealed record Or(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) || Right.Eval(tags); }

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using System; using System.Collections.Generic; using System.Linq; IExpr rule = new Or(new HasTag("article"), new And(new HasTag("video"), new HasTag("captioned"))); foreach (var tags in new[] { new HashSet<string> { "article" }, new HashSet<string> { "video" }, new HashSet<string> { "video", "captioned" } }) Console.WriteLine(rule.Eval(tags)); // Different expression kinds evaluate the same input to a boolean result. interface IExpr { bool Eval(IReadOnlySet<string> tags); } sealed record HasTag(string Tag) : IExpr { public bool Eval(IReadOnlySet<string> tags) => tags.Contains(Tag); } // Delegate to child expressions and own only the combination rule. sealed record And(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) && Right.Eval(tags); } sealed record Or(IExpr Left, IExpr Right) : IExpr { public bool Eval(IReadOnlySet<string> tags) => Left.Eval(tags) || Right.Eval(tags); }

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Content blocks 24 → 29-30

Each example is a separate .NET 10 console program.

Each C# example is a separate .NET 10 console program. Compile the TypeScript examples for ES2022 or later and run the emitted JavaScript with Node.js.