namespace Nemerle.IO
{
    public macro PathExpander(path)
    syntax ("~", path)
    {
        <[
            def getEnvironmentVariable(name) {Environment.GetEnvironmentVariable(name) ?? throw Exception($"Environment variable $name is not defined")}
            
            match ($[x | x in $path.Split(IO.Path.DirectorySeparatorChar)])
            {
                | headNode :: tailNodes =>
                    ((regexp match (headNode)
                    {
                        | @"^~$"          => Environment.GetEnvironmentVariable("USERPROFILE")
                        | @"^#(?<sf>.*)#" => Environment.GetFolderPath(Enum.Parse(typeof(Environment.SpecialFolder), sf, false) :> Environment.SpecialFolder)
                        | @"^%(?<ev>.*)%" => getEnvironmentVariable(ev)
                        | _               => if (!IO.Path.IsPathRooted(headNode)) IO.Path.Combine(IO.Directory.GetCurrentDirectory(), headNode) else $"$(headNode)$(Path.DirectorySeparatorChar)"
                    }) :: tailNodes).Fold(headNode, fun(node, path)
                    {
                        IO.Path.Combine(path,
                            regexp match(node) 
                            {
                                | @"^%(?<ev>.*)%" => getEnvironmentVariable(ev)
                                | _ => node
                            }
                        )
                    })
            }
        ]>
    }
}

WriteLine(~"Nemerle.dll");
WriteLine(~(@"~\Links\desktop.lnk"));
WriteLine(~"#ProgramFiles#\\Nemerle\\docs");
WriteLine(~"%SystemRoot%\\System32\\drivers\\etc\\hosts"));
WriteLine(~"C:\\Temp\\%PROCESSOR_ARCHITECTURE%");
def path = @"~\desktop.ini";
WriteLine(~path);

C:\Users\VKochetkov\Documents\Visual Studio 2008\Projects\PathExpansion\PathExpander\bin\Debug\Nemerle.dll
C:\Users\VKochetkov\Links\desktop.lnk
C:\Program Files\Nemerle\docs
C:\Windows\System32\drivers\etc\hosts
C:\Temp\x86
C:\Users\VKochetkov\desktop.ini

N>foo :: a -> b
N>foo x = foo [x]
N>

class A<T> {
    public A<T> Value { get; set}
}