问题
Please give an example of how to create new type (say, two types Cartesian product) in F# at runtime with reflection?
UPDATE
I am looking for a language with first class types. I was told F# can this. I tried nothing since didn't learned F# yet. I just want to see how it's made.
回答1:
The following F# code takes 2 sequences of values (rank and suit in the example) and returns the cartesian product as a sequence of pairs (cards), using a pair type dynamically generated at runtime using Reflection:
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
回答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
来源：https://stackoverflow.com/questions/14549244/how-to-create-new-type-at-runtime-in-f