Expr< int>的F#匹配模式 [英] F# match pattern for Expr<int>

问题描述

我尝试使用以下代码找到匹配的正确模式并运行Expr<int>:

I try to find the correct pattern to match and run an Expr<int> using the below code:

open System.Linq

open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Quotations.Patterns

let runSelectQuery (q:Expr<IQueryable<'T>>) = 
    match q with
    | Application(Lambda(builder, Call(Some builder2, miRun, [Quote body])), queryObj) ->
        query.Run(Expr.Cast<Microsoft.FSharp.Linq.QuerySource<'T, IQueryable>>(body))
    | _ -> failwith "Cannot run this query %s" (q.ToString())

let runCountQuery (q:Expr<int>) = 
    match q with
    | Application(Lambda(builder, Call(None, miRun, [builder2, Quote body])), queryObj) ->
        query.Run(Expr.Cast<int>(body))
    | _ -> failwith "Cannot run this query %s" (q.ToString())

let countQuery source filter =
    let filter = match filter with | Some filter -> filter | _ -> <@ fun _ -> true @>
    <@ query { for item in source do
               where ((%filter) item)
               count } @>

runSelectQuery正确匹配Expr<IQueryable<'T>>模式.但是，我找不到与我的通用计数查询Expr<int>

The runSelectQuery correctly matches the Expr<IQueryable<'T>> pattern. However, I cannot find the correct pattern to match my generic count query Expr<int>

我从countQuery签名派生的代码中的模式给了我一个

The pattern in the code I derived from the signature of countQuery gives me a:

该表达式应具有类型 Expr，但是这里有类型 'a *'b

This expression was expected to have type Expr but here has type 'a * 'b

推荐答案

找到了！愚蠢的是，我首先尝试使用逗号分隔的模式来匹配数组模式(C#中的列表定界符)，显然在F#中不起作用，抱怨它不是列表，而是tupple，因此不是Rex.

Found it! Stupidly I first tried to match the array pattern using a comma separated pattern (as is the list delimiter in C#), that obviously did not work in F# complaining the it was not a list but a tupple and thus not a Rex.

要再次匹配一个int结果或任何'T结果:

To match agains an int result or any 'T result:

let runQueryToQueryable (q:Expr<IQueryable<'T>>) = 
    match q with
    | Application(Lambda(builder, Call(Some builder2, miRun, [Quote body])), queryObj) ->
        query.Run(Expr.Cast<Microsoft.FSharp.Linq.QuerySource<'T, IQueryable>>(body))
    | _ -> failwith "Cannot run this query %s" (q.ToString())

let runQueryToType (q:Expr<'T>) = 
    match q with
    | Application(Lambda(builder, Call(None, miRun, [builder2; Quote body])), queryObj) ->
           query.Run(Expr.Cast<'T>(body))
    | _ -> failwith "Cannot run this query %s" (q.ToString())

像魅力一样工作.

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