Thanks :) The REPL example with the error has a deliberate typo (int instead of Int) to show how multi-line editing works in the REPL - is that the part you're asking about?
$ swift
Welcome to Swift! Type :help for assistance.
1> 3.14 * 2.5 * 2.5
$R0: Double = 19.625
2> println("The area of the circle is \($R0)")
The area of the circle is 19.625
3>
4> $R0 * 2
$R1: Double = 39.25
5> println("And double that is \($R1)")
And double that is 39.25
6>
What's Double? Trying to follow this (which didn't make sense to me because I didn't know the history stuff was there) simply caused error messages.
I did find the first few examples with the REPL to be a bit confusing. Were you using a function before it was defined?