17 May 2009

The Smileys in Scala

or: a visual interpretation of some of the syntax of the Scala programming language, with the aim of providing an aide memoire.

The Happy Walrus     : =>

(Yeah, I needed to include the : to make this work).

“The walrus prefers shallow shelf regions and forages on the sea bottom.” And like the walrus, in Scala the symbol for by name parameters allows the shallow surface parameter to forage deep inside your code.

An example from Beginning Scala of a method that keeps appending the results of a block of code until the test is true:

def bmap[T](test: => Boolean)(block: => T): List[T] = {
 val ret = new ListBuffer[T]
 while (test) ret += block

Here we see two walruses, test and block, neither of which are evaluated as parameters until you dive into the body of the method.

The Scissors     >:

Putting a lower bounds on a type is rather like cutting off the option of passing a subclass. That is, anything to the left of the scissors in…

def doStuff[B >: T](p: B) = ...

… is snipped away if the type to the left is a subclass (“below”) the type on the right. I find it helps to mentally rotate 90 degrees anti-clockwise to see the scissors cutting B if it’s “below” T in the class hierarchy.

Chapter 19 of Programming in Scala has the details, as does 8.3 of Scala By Example (PDF).

Let’s wield the scissors, by considering a manager who can’t handle detail: they can think about fruit and apples, but nothing more specific…

scala> case class Fruit()
defined class Fruit

scala> case class Apple() extends Fruit
defined class Apple

scala> case class Pippin() extends Apple
defined class Pippin

scala> class Manager[A >: Apple]
defined class Manager

scala> new Manager[Fruit]
res2: Manager[Fruit] = Manager@3b2000a5

scala> new Manager[Apple]
res3: Manager[Apple] = Manager@276c9124

scala> new Manager[Pippin]
:11: error: type arguments [Pippin] do not conform to class Manager's type parameter bounds [A >: Apple]

The Fussy Bird     <:

When it comes to…

A <: B 

…the fussy bird knows that the morsel on the left is the special, tastier class when compared to the big old lump of class to the right. In Scala it signifies an upper bound, that A must be the same or a subtype (specialism) of B. And special means tasty, so no wonder she’s got her beak and eyes pointing left towards the tasty class.

scala> case class Stuff()
defined class Stuff

scala> case class Food() extends Stuff
defined class Food

scala> case class TastyFood() extends Food
defined class TastyFood

scala> def peck[A <: Food](p: A) = println("yum")
peck: [A <: Food](A)Unit

scala> peck(Food())

scala> peck(TastyFood())

scala> peck(Stuff())
:10: error: inferred type arguments [Stuff] do not conform to method peck's type parameter bounds [A <: Food] peck(Stuff())

The bird is so fussy she’ll even be happy with Nothing.

So that’s the three I’ve had to work to remember. I hope I’ve got that far enough in the direction of correct to be useful. Someone will have to sort out stories for <% (view bounds) and the rest of the syntax you might see such as /:.