表达式<Func<T，bool>来自 F# func [英] Expression<Func<T, bool>> from a F# func

问题描述

在 linq 中，.Where 需要一个 Expression> 谓词，我可以在 F# 中将其写为

in linq, .Where takes a Expression> predicate, which I can write in F# as

<@ fun item:'a -> condition @>    // Expr<'a -> bool>

我使用 FSharp.Powerpack 从引用中构建表达式，但它给我的是 MethodCallExpression.深入研究，powerpack 代码正确构建了 lambda，但将其包装在 Convert 调用中(为什么会这样?).我想知道将参数转换为方法调用(一个 lambda)是否最终会给我我需要的表达式.

I'm using FSharp.Powerpack to build the expression from a quotation, but what it gives me is a MethodCallExpression. Looking deep, the powerpack code builds the lambda correctly, but wraps it in a Convert call (why is that?). I wonder if casting the argument to the method call (a lambda) would finally give me the Expression> I need.

所以问题是为什么要调用 Convert，以及如何实际获得带有 Func 签名的 lambda.

So the question is why the Convert call, and how to actually get the lambda with the Func signature.

推荐答案

我一时想不起来我是在哪里找到这段代码的，但这是我用来转换 Expr<'a ->'b> 到 Expression>.希望这能解决您的问题.

I can't remember off the top of my head where I found this bit of code, but this is what I use to convert an Expr<'a -> 'b> to Expression<Func<'a, 'b>>. Hopefully this will solve your problem.

open System
open System.Linq.Expressions
open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Linq.QuotationEvaluation

let toLinq (expr : Expr<'a -> 'b>) =
  let linq = expr.ToLinqExpression()
  let call = linq :?> MethodCallExpression
  let lambda = call.Arguments.[0] :?> LambdaExpression
  Expression.Lambda<Func<'a, 'b>>(lambda.Body, lambda.Parameters) 

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