Pattern Matching

  The basic pattern matching construct is the match statement. A match statement can be seen as a generalization of C++'s switch statement, in which the expression that follows a case can be a general complex pattern.

Its basic syntax is as follows:

Match	::=	Match_Mode ( Exp ) and ¼ and ( Exp )
		{ case Match_Rule ¼ case Match_Rule }	variant 1
	|	Match_Mode ( Exp ) and ¼ and ( Exp )
		{ Match_Rule  - ¼ - Match_Rule }	variant 2
	|	Match_Mode ( Exp ) and ¼ and ( Exp ) of	variant 3
		Match_Rule  - ¼ - Match_Rule
		end match ;
Match_Mode	::=	match [ while ]
	|	matchall [ while ]
Match_Rule	::=	Pat [ Guard ] [ Cost ] : Match_Action
Guard	::=	if Exp
	|	where Exp
	|	- Exp
Cost	::=	\ Exp	cost expression
Match_Action	::=	{ Code }
	|	Code	for variant 1 only

The 3 syntactic variants of the match statement are equivalent in meaning. A match statement takes a list of match objects and a set of pattern matching rules. The first rule that matches the pattern completely will be executed. Notice that guards can be placed on each rule, and a rule is only applicable if the guards evaluate to true.

 
• Cost minimization
• Match all rules
• Repetition
• Pattern syntax