Make F# example more idiomatic (#46)

examples/Microsoft.Spark.FSharp.Examples/Program.fs

@@ -5,23 +5,18 @@
 module Microsoft.Spark.Examples.Main
 
 open System
-open System.Collections.Generic
-open System.Linq
 open System.Reflection
-open System.Runtime.InteropServices
 
-let printUsage (examples : IEnumerable<string>) =
+let printUsage (examples : string seq) =
     let assemblyName = Assembly.GetExecutingAssembly().GetName().Name
     printfn "Usage: %s <example> <example args>" assemblyName
 
-    if examples.Any() then
+    if not (Seq.isEmpty examples) then
         printfn "Examples:\n\t*%s" (examples |> String.concat "\n\t*")
     printfn "\n'%s <example>' to get the usage info of each example." assemblyName
 
-let tryFindExample (examples: IEnumerable<string>, search: string, [<Out>] found : string byref) =
-    found <- examples.FirstOrDefault(fun e ->
-        e.Equals(search, StringComparison.InvariantCultureIgnoreCase))
-    not (String.IsNullOrWhiteSpace(found))
+let tryFindExample search (examples: string seq) =
+    examples |> Seq.tryFind (fun e -> e.Equals(search, StringComparison.InvariantCultureIgnoreCase))
 
 [<EntryPoint>]
 let main args =
@@ -32,29 +27,29 @@ let main args =
     // name of the type after the rootNamespace.
     let examples =
         Assembly.GetExecutingAssembly().GetTypes()
-            .Where(fun t -> 
+            |> Seq.filter (fun t -> 
                 typeof<IExample>.IsAssignableFrom(t) &&
                 not t.IsInterface &&
                 not t.IsAbstract &&
                 t.Namespace.StartsWith(rootNamespace) &&
                 ((t.Namespace.Length = rootNamespace.Length) ||
                  (t.Namespace.[rootNamespace.Length] = '.')))
-            .Select(fun t -> t.FullName.Substring(rootNamespace.Length + 1))
+            |> Seq.map (fun t -> t.FullName.Substring(rootNamespace.Length + 1))
 
     match args with
     | [||] ->
-        printUsage(examples)
+        printUsage examples
         1
     | _ ->
-        let mutable exampleName = String.Empty
-        if not (tryFindExample(examples, args.[0], &exampleName)) then
-            printUsage(examples)
+        match examples |> tryFindExample (args |> Array.head) with
+        | None ->
+            printUsage examples
             1
-        else
-            let exampleArgs = args.Skip(1).ToArray()
+        | Some exampleName ->
+            let exampleArgs = args |> Array.skip 1
             let exampleType =
                 Assembly.GetExecutingAssembly()
                     .GetType(sprintf "%s.%s"  rootNamespace exampleName)
             let instance = Activator.CreateInstance(exampleType)
             let method = exampleType.GetMethod("Run")
-            method.Invoke(instance, [|exampleArgs|]) :?> int
+            method.Invoke(instance, [|exampleArgs|]) :?> int