Micro frameworks are popular now, so I'll go nano framework :-). This is all the code:
/**
* `undefined()` pretends to be able to produce a value of any type `T` which can
* be very useful whilst writing a program. It happens that you need a value
* (which can be a function as well) of a certain type but you can't produce it
* just yet. However, you can always temporarily replace it by `undefined()`.
*
* Inspired by Haskell's
* [undefined](http://hackage.haskell.org/package/base-4.7.0.2/docs/Prelude.html#v:undefined).
*
* Invoking `undefined()` will crash your program.
*
* Some examples:
*
* - `let x : String = undefined()`
* - `let f : String -> Int? = undefined("string to optional int function")`
* - `return undefined() /* in any function */`
* - `let x : String = (undefined() as Int -> String)(42)`
* - ...
*
* What a crash looks like:
*
* `fatal error: undefined: main.swift, line 131`
*
*/
public func undefined<T>(hint: String = "", file: StaticString = #file, line: UInt = #line) -> T {
let message = hint == "" ? "" : ": \(hint)"
fatalError("undefined \(T.self)\(message)", file:file, line:line)
}
See my slides about undefined() from my Further Leveraging the Type System talk at Swift Summit 2015.