reflection - 如何在 F# 运行时创建新类型？

Question

请举例说明如何在运行时通过反射在 F# 中创建新类型（例如，两种类型的笛卡尔积）？

更新

我正在寻找一种具有一流类型的语言。有人告诉我 F# 可以。我什么都没试过，因为还没有学过 F#。我只是想看看它是如何制作的。

Answer 1

以下 F# 代码采用 2 个值序列（示例中的等级和花色），并使用在运行时使用反射动态生成的对类型将笛卡尔积作为对（卡片）序列返回：

open System
open System.Reflection
open System.Reflection.Emit
open Microsoft.FSharp.Reflection

/// Creates a dynamic module via reflection
let createModule () =
    let name = Guid.NewGuid().ToString()
    let d = AppDomain.CurrentDomain
    let a = d.DefineDynamicAssembly(AssemblyName(name), AssemblyBuilderAccess.Run)
    a.DefineDynamicModule(name)
/// Creates a dynamic pair type using the specified x and y types
let createPairType (x:Type, y:Type) =
    let m = createModule()
    let t = m.DefineType("Pair", TypeAttributes.Public ||| TypeAttributes.Class)
    let x = t.DefineField(x.Name, x, FieldAttributes.Public)
    let y = t.DefineField(y.Name, y, FieldAttributes.Public)
    t.CreateType()
/// Creates a pair value using the specified pair type
let createPairValue (pairType:Type) (x:'X, y:'Y) =
    let instance = Activator.CreateInstance(pairType)
    pairType.GetField(typeof<'X>.Name).SetValue(instance, x)
    pairType.GetField(typeof<'Y>.Name).SetValue(instance, y)
    instance
/// Creates a cartesian product 
let createCartesianProduct (xs:'X seq, ys:'Y seq) =
    let pairType = createPairType (typeof<'X>,typeof<'Y>) 
    seq { for x in xs do for y in ys -> createPairValue pairType (x, y) }
/// Defines dynamic lookup operator for accessing a named field
let inline (?) (x:obj) name = x.GetType().GetField(name).GetValue(x)
/// Card suit discriminated union type
type Suit = Club | Diamond | Heart | Spade
/// Card rank discriminated union type 
type Rank = | One | Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten
            | Jack | Queen | King | Ace
/// Gets union case values
let getUnionValues<'T>() = 
    FSharpType.GetUnionCases(typeof<'T>) 
    |> Seq.map (fun x -> FSharpValue.MakeUnion(x,[||]) :?> 'T)
let ranks, suits = getUnionValues<Rank>(), getUnionValues<Suit>()
/// Sequence of dynamically generated pairs
let cards = createCartesianProduct (ranks, suits)
// Paste this into F# interactive to print the generated cards
for card in cards do printfn "%A %A" card?Rank card?Suit

Answer 2

Taken from my (non-free) article Structural Typing in the F#.NET Journal:

The following createType function creates a new .NET assembly, new module and new public class type of the given name:

> let createType typeName =
    let name = System.Reflection.AssemblyName(Name="tmpAssembly")
    let run = System.Reflection.Emit.AssemblyBuilderAccess.Run
    let builder = System.Threading.Thread.GetDomain().DefineDynamicAssembly(name, run)
    let mdl = builder.DefineDynamicModule "tmpModule"
    let attrs = TypeAttributes.Public ||| TypeAttributes.Class
    mdl.DefineType(typeName, attrs);;
val createType : string -> TypeBuilder