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POF# design pattern
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copied!<p>Lets say I'm building a parser for a domain-specific language in F#.</p> <p>I've defined a discriminated union to represent expressions:</p> <pre><code> type Expression = | Equality of Expression*Expression | NonEquality of Expression*Expression | Or of Expression*Expression | And of Expression*Expression | If of Expression*Expression | IfElse of Expression*Expression*Expression | Bool of bool | Variable of string | StringLiteral of string </code></pre> <p>Now, I've built up an AST of type <code>Expression</code> and want to generate code for it. I have one function which does type inference and type checking on an expression.</p> <p>It's defined like</p> <pre><code> let rec InferType expr = match expr with | Equality(e1,e2) -> CheckTypes (InferType e1) (InferType e2) | Or(e1,e2) -> CheckTypes (InferType e1) (InferType e2) | And(e1,e2) -> CheckTypes (InferType e1) (InferType e2) ... </code></pre> <p>And I have another function to generate code which follows a similar pattern: Take an expression, write pattern-matching statements for each item in the union.</p> <p>My question is: Is this the idiomatic way to do it in F#?</p> <p>It seems to me that it would be cleaner if each member of the union defined its own <code>InferType</code> and <code>GenerateCode</code> locally with it.</p> <p>If I were using C#, I would define some abstract base class called <code>Expression</code> with virtual methods for <code>InferType</code> and <code>GenerateCode</code> and then override them in each subclass.</p> <p>Is there any other way to do this?</p>

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