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    <p>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:</p> <pre><code>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&lt;'X&gt;.Name).SetValue(instance, x) pairType.GetField(typeof&lt;'Y&gt;.Name).SetValue(instance, y) instance /// Creates a cartesian product let createCartesianProduct (xs:'X seq, ys:'Y seq) = let pairType = createPairType (typeof&lt;'X&gt;,typeof&lt;'Y&gt;) seq { for x in xs do for y in ys -&gt; 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&lt;'T&gt;() = FSharpType.GetUnionCases(typeof&lt;'T&gt;) |&gt; Seq.map (fun x -&gt; FSharpValue.MakeUnion(x,[||]) :?&gt; 'T) let ranks, suits = getUnionValues&lt;Rank&gt;(), getUnionValues&lt;Suit&gt;() /// 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 </code></pre>
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    1. POHow to create new type at runtime in F#?
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    1. USPhillip Trelford
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      1. VTUpMod
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    1. COIncredible! Why is it so long???
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      1. USSuzan Cioc
    2. COThe built-in .Net API for generating types dynamically via reflection is quite verbose. If you had a practical application for this you'd probably hide it behind a library.
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      1. USPhillip Trelford
    3. COThere are only a couple of cases where I've found it useful to generate types dynamically in .Net (1) for mapping a plain text DSL to types for debugging in the [TickSpec](http://tickspec.codeplex.com) OSS project (2) for generating mock types against interfaces for unit testing in the [Foq](http://foq.codeplex.com) OSS project. To generate cards without reflection as a sequence of tuples simply do seq { for suit in suits do for rank in ranks do yield rank, suit }
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      1. USPhillip Trelford
 

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