Introduction to Scala Part3: Option & Pattern Matching

Pattern Matching

Pattern matching is a generalization of switch from C/Java to class hierarchies. It’s expressed in Scala using the keyword match.

Examples:

def eval(e: Expr): Int = e match {
    case Number(n) => n
    case Sum(e1, e2) => eval(e1) + eval(e2)
}

Rules

• Match is followed by a sequence of cases
• Each case associates an expression to a pattern
• An exception MatchError is thrown if no pattern matches the value of the selector

Pattern forms

Patterns are constructed from:

• Constructors : Number, Sum, etc.
• Variables : n, e1, e2, etc.
• Wildcard patterns : _
• Constants : 1 , true

Variables always begin with a lowercase letter. The same variable name can only appear once in a pattern (Sum(x, x) is not a legal pattern)

Constructors and the names of constants begin with a capital letter, with the exception of the reserved words: null, true ,false