R – How to use the F# Reflection library

%freflection

I am trying to follow this example (from p137 of Rob Pickering's "Foundations of F#" book) but I can't get it to work with the latest F# CTP.

I appear to be missing the definition of 'Value' on the 3rd line where it does

Value.GetInfo(x)

This generates :

error FS0039: The namespace or module 'Value' is not defined.

Can anyone tell me where this is coming from or what the new syntax is if this is now done differently? (be gentle – this is my first play with F#)

Here's the example I am working from:-

#light
open Microsoft.FSharp.Reflection
let printTupleValues x =
    match Value.GetInfo(x) with
    | TupleValue vals ->
    print_string "("
    vals
    |> List.iteri
        (fun i v ->
            if i <> List.length vals - 1 then
                Printf.printf " %s, " (any_to_string v)
            else
                print_any v)
    print_string " )"
    | _ -> print_string "not a tuple"

printTupleValues ("hello world", 1)

Best Answer

The F# reflection library was rewritten for either Beta 1 or the CTP. Here is your code slightly changed to use the new library, and to avoid using the F# PlusPack (print_string is for OCaml compatibility).

open Microsoft.FSharp.Reflection

let printTupleValues x =
    if FSharpType.IsTuple( x.GetType() ) then
        let s =
            FSharpValue.GetTupleFields( x )
            |> Array.map (fun a -> a.ToString())
            |> Array.reduce (fun a b -> sprintf "%s, %s" a b)
        printfn "(%s)" s
    else 
        printfn "not a tuple"

printTupleValues ("hello world", 1)

Related Solutions

Java – reflection and why is it useful

The name reflection is used to describe code which is able to inspect other code in the same system (or itself).

For example, say you have an object of an unknown type in Java, and you would like to call a 'doSomething' method on it if one exists. Java's static typing system isn't really designed to support this unless the object conforms to a known interface, but using reflection, your code can look at the object and find out if it has a method called 'doSomething' and then call it if you want to.

So, to give you a code example of this in Java (imagine the object in question is foo) :

Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);

One very common use case in Java is the usage with annotations. JUnit 4, for example, will use reflection to look through your classes for methods tagged with the @Test annotation, and will then call them when running the unit test.

There are some good reflection examples to get you started at http://docs.oracle.com/javase/tutorial/reflect/index.html

And finally, yes, the concepts are pretty much similar in other statically typed languages which support reflection (like C#). In dynamically typed languages, the use case described above is less necessary (since the compiler will allow any method to be called on any object, failing at runtime if it does not exist), but the second case of looking for methods which are marked or work in a certain way is still common.

Update from a comment:

The ability to inspect the code in the system and see object types is not reflection, but rather Type Introspection. Reflection is then the ability to make modifications at runtime by making use of introspection. The distinction is necessary here as some languages support introspection, but do not support reflection. One such example is C++

C# – How to get the path of the assembly the code is in

I've defined the following property as we use this often in unit testing.

public static string AssemblyDirectory
{
    get
    {
        string codeBase = Assembly.GetExecutingAssembly().CodeBase;
        UriBuilder uri = new UriBuilder(codeBase);
        string path = Uri.UnescapeDataString(uri.Path);
        return Path.GetDirectoryName(path);
    }
}

The Assembly.Location property sometimes gives you some funny results when using NUnit (where assemblies run from a temporary folder), so I prefer to use CodeBase which gives you the path in URI format, then UriBuild.UnescapeDataString removes the File:// at the beginning, and GetDirectoryName changes it to the normal windows format.

Best Answer

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Java – reflection and why is it useful

C# – How to get the path of the assembly the code is in

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