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F# Metaprogramming part 2: Dynamic synthesis of executable F# code

April 1, 2012

No, this is not an April 1 Fool’s day joke. Someone asked for this on Stack Overflow few days ago. As this is a good complementary to my JITting foray, I spent some time today on this exciting matter and was able to come up with reasonably succinct working sample. I simply repost here my own SO answer.

The desired can be achieved using F# CodeDom provider. A minimal runnable snippet below demonstrates the required steps. It takes an arbitrary presumably correct F# code from a string and tries to compile it into an assembly file. If successful, then it loads this just synthesized assembly from the correspondent dll file and invokes a known function from there, otherwise it shows what’s the problem with compiling the code.

open System 
open System.CodeDom.Compiler 
open Microsoft.FSharp.Compiler.CodeDom 

// Our (very simple) code string consisting of just one function: unit -> string 
let codeString =
    "module Synthetic.Code\n    let syntheticFunction() = \"I've been compiled on the fly!\""
// Assembly path to keep compiled code
let synthAssemblyPath = "synthetic.dll"

let CompileFSharpCode(codeString, synthAssemblyPath) =
    use provider = new FSharpCodeProvider() 
    let options = CompilerParameters([||], synthAssemblyPath) 
    let result = provider.CompileAssemblyFromSource( options, [|codeString|] ) 
    // If we missed anything, let compiler show us what's the problem
    if result.Errors.Count <> 0 then  
        for i = 0 to result.Errors.Count - 1 do
            printfn "%A" (result.Errors.Item(i).ErrorText)
    result.Errors.Count = 0

if CompileFSharpCode(codeString, synthAssemblyPath) then
    let synthAssembly = Reflection.Assembly.LoadFrom(synthAssemblyPath) 
    let synthMethod  = synthAssembly.GetType("Synthetic.Code").GetMethod("syntheticFunction") 
    printfn "Success: %A" (synthMethod.Invoke(null, null))
else
    failwith "Compilation failed"

If anyone wants to play with this toy, two references will be needed: to FSharp.Compiler.dll and to FSharp.Compiler.CodeDom.dll. Enjoy!

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