F#引用对象图

Question

在C#中,我可以使用表达式树很容易地创建对象图的字符串表示.

public static string GetGraph<TModel, T>(TModel model, Expression<Func<TModel, T>> action) where TModel : class
{
            var method = action.Body as MethodCallExpression;
            var body = method != null ? method.Object != null ? method.Object as MemberExpression : method.Arguments.Any() ? method.Arguments.First() as MemberExpression : null : action.Body as MemberExpression;
            if (body != null)
            {
                string graph = GetObjectGraph(body, typeof(TModel))
                return graph;
            }
            throw new Exception("Could not create object graph");
}

在F#中我一直在看引文试图做同样的事情,并且无法弄明白.我曾尝试使用PowerPack库将引用转换为Expression,但到目前为止还没有运气,而且互联网上的信息在这个主题上看起来相当稀少.

如果输入是:

let result = getGraph myObject <@ myObject.MyProperty @>

输出应为"myobject.MyProperty"

Answer 1

您可以在fsi会话中看到从引号表达式中获得的内容:

> let v = "abc"
> <@ v.Length @>;;
val it : Expr<int>
= PropGet (Some (PropGet (None, System.String v, [])), Int32 Length, [])

> <@ "abc".Length @>;;
val it : Expr<int>
= PropGet (Some (Value ("abc")), Int32 Length, [])

您可以找到可用于解析qoutations的所有活动模式的描述

手动\ FSharp.Core\Microsoft.FSharp.Quotations.Patterns.html

在您的F#安装目录下或在msdn站点下

有很好的Chris Smith的书"Programming F#",章节名为"Quotations":)

所以,毕竟,只是尝试编写简单的解析器:

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

let rec getGraph (expr: Expr) =
  let parse args =
    List.fold_left (fun acc v -> acc ^ (if acc.Length > 0 then "," else "") ^ getGraph v) "" args
  let descr s = function
    | Some v -> "(* instance " ^ s ^ "*) " ^ getGraph v
    | _ -> "(* static " ^ s ^ "*)"
  match expr with
  | Int32 i -> string i
  | String s -> sprintf "\"%s\"" s
  | Value (o,t) -> sprintf "%A" o
  | Call (e, methodInfo, av) ->
    sprintf "%s.%s(%s)" (descr "method" e) methodInfo.Name (parse av)
  | PropGet(e, methodInfo, av) ->
    sprintf "%s.%s(%s)" (descr "property" e) methodInfo.Name (parse av)
  | _ -> failwithf "I'm don't understand such expression's form yet: %A" expr

PS当然,您需要一些代码才能将AST转换为人类可读的格式.