How to use map?

[Lemonfiend via Digitalmars-d-learn]

> The code you were trying to write:
>
> struct Foo(T) {
>     T t;
> }
>
> void main() {
>     import std.stdio, std.algorithm, std.array;
>
>     float[] vals = [1.1, 2.1, 3.1, 4.1];
>     auto arr = vals.map!(Foo!float).array;
>     arr.writeln;
> }

Sorry, my example had an unneeded template. Simply this (and also 
your code)
---
struct S
{
     float f;
}
---
still results in an undefined identifier.