Comments on Scala Blog: Roman numerals in Scala

In the definition of "unfold""case Some(r, v)" sho...

2008-04-02T04:59:00.000-07:00

In the definition of "unfold"
"case Some(r, v)" should actually be
"case Some((r, v))" (extra parenthesis).

Incidentally, there is a deep category-theoretical...

2008-01-30T17:10:00.000-08:00

Incidentally, there is a deep category-theoretical foundation to the unfold operation: it's formally known as a anamorphism (or sometimes the lens operator).

The classic paper Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire provides great insight into several standard recursive operators, and should be considered required reading for serious functional programmers.

I'm surprised to hear that Scala lacks unfold: I hope it will appear in a future release!

Interesting piece of code, I found it useful to g...

2008-01-23T10:13:00.000-08:00

Interesting piece of code, I found it useful to get my head around it. And after taking a quick look at Daniel's deromanize function, I tried to make it myself without peeking. It took a few tries... and peeks... but well, that's how you learn! At some point I'll be able to write stuff like that more naturally.
My favorite parts from ScalaByExample were the exercises... can you guys come up with some in this blog?? That's valuable training!

def deromanize(roman : String) : int = numeral...

2008-01-16T23:47:00.000-08:00

def deromanize(roman : String) : int =
numerals.filter(roman startsWith _._1).sort(_._1.length > _._1.length) match {
case (s, v) :: _ => v + deromanize(roman.substring(s.length))
case Nil => 0
}

woo!

Ah nm.. got it: The first '_' is the normal scala ...

2008-01-14T09:48:00.000-08:00

Ah nm.. got it:

The first '_' is the normal scala shorthand for the first parameter of the anonymous function.

The second '_' is part of the '_2' val, that returns the second part of the tuple. The '_' in '_2' threw me off.

Cool stuff. In the definition of "next", could you...

2008-01-14T09:33:00.000-08:00

Cool stuff. In the definition of "next", could you explain how the '_._2 <= in' part works? It's the only part thats a little unclear to me..