Neat Samples: MEF in F# Scripts - Sharp Things - Site Home

Jomo Fisher—I posted yesterday about using MEF in F# programs. In the comments Oldrich asked if it was possible to do the same thing in F# scripts. It is indeed possible. It actually looks like a really powerful combination because you can naturally extend your scripts just by adding new #load directives at the top.

There are three pieces to this sample. You have to break your interfaces out into a separate .fsx. Realistically, you’d probably do this in a real compiled F# program too so this isn’t a huge burden on simplicity.

Code Snippet

// MefInterfaces.fsx
// Expose an interface
type ITastyTreat =
    abstract Description : string

Code Snippet

// ChocolateCookie.fsx
#r "System.ComponentModel.Composition"
#load "MefInterfaces.fsx"
namespace ChocolateCookie
open MefInterfaces
open System.ComponentModel.Composition
[<Export(typeof<ITastyTreat>)>]
type ChocolateCookie() = 
    interface ITastyTreat with 
        member __.Description = "a delicious chocolate cookie"

Code Snippet

// MefHost.fsx
#load "ChocolateCookie.fsx"
open System.Reflection
open System.IO
open System.ComponentModel.Composition
open System.ComponentModel.Composition.Hosting
open MefInterfaces
// Set up MEF
let catalog = new AggregateCatalog()
let assemblyCatalog = new AssemblyCatalog(Assembly.GetExecutingAssembly())
let container = new CompositionContainer(catalog)
catalog.Catalogs.Add(assemblyCatalog)
// Jar that will contain tasty treats
type TreatJar() = 
    [<ImportMany(typeof<ITastyTreat>)>] 
    let cookies : seq<ITastyTreat> = null
    member __.EatTreats() =
        cookies |> Seq.iter(fun tt->printfn "Yum, it was a %s" tt.Description)
let jar = TreatJar()
container.ComposeParts(jar)
jar.EatTreats()

Thanks Oldrich, for giving me an excuse to try this out.

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