Saturday, August 8, 2009

Programming In Scala: Chapter#4,5 notes

public is scala's default access level.

Method parameters in scala are val, they can't be reassigned.

In the absence of any returned statement, scala returns the last expression value computed by the method. The recommeded or the functional way is not to have explicit return statements and to definitely avoid multiple return statements. This philosophy encourages you to make small methods that just do one thing and not many.

Braces around the method definition are optional if there is only one expression inside the method body.

If you have a method whose return type is Unit, you can write it like following(removing the = sign) so that it looks like a procedure and clarifies the intention that its there for side effects only..
def add(a:Int) {sum+=a}

One thing to notice here is that whenever you write a function in procedural style(that is without equal sign), its infered return type is definitely going to be Unit no matter what the body contains.
A semicolon is *must* if you write multiple statements on a single line.

The rules of semicolon inference:
1. The line in question ends in a word that would not be legal as the end of a statement, such as a period or as an infix operator.
2. The next line begins with a word that cannot start with a statement.
3. The line ends while inside parentheses(...) or brackets [...], because these cannot contain multiple statements anyway.

Sigleton Objects: Scala classes can't have static fields or method, instead there can be singleton object whose definition looks exactly like scala classes with class replaced with keyword object. If there is a class with same name then singleton object is called companion object of that class, similarly that class is called companion class of this singleton object. Companion class/object can access private members of each other. A singleton object and the companion class *MUST* be defined in the same source file.

Other than the value types(corresponding to every primitive type in java) such as Byte, Short, Int, Long, Float, Double and Boolean, scala has a type called symbol which is like scheme symbols and literal representation is same as scheme symbols, they start with single quote e.f.'symbol

Symbols are interned, if you write the same symbol twice, they both refer to the same symbol object.

unary_x, where x = +/-/!/~ can be used in prefix notation that is 2.unary_- is same as -2.

If a method takes no arguments, it can be written in postfix notation, that is s.toStrin() can be written as "s toString".

A == B in scala first checks to see if A is null, if its not then it checks
A.equals(B). In java '==' compares referential equality which here is done with method eq(this only applies to the objects which can be directly mapped into java objects)

Scala doesn't have operators, so how does 2+2*7 return 16. The answer is that precedence is based on the first letter of the method name and method starting with * have higher precedence than methods starting with +.

The associativity(when there are multiple methods with same precendence) is determined by the last character of the method name.

For every basic scala type, there is a rich wrapper that provides more utility methods on it.

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