Add match expressions to entity

design/spec.md

@@ -256,7 +256,7 @@ If the type of the union matches the type of the declaration in the case, the
 expression is executed and the value of the union is made available to it
 through the declaration. If the value of the match expression is used, all
 possible types in the union must be accounted for. It may be assigned to any
-type that satisfied
+type that satisfies the assignment rules of its first case.
 ### Loop
 Loop is a control flow expression that repeats an expression until a break
 statement is called from within it. The break statement must be given a value

entity/expression.go

@@ -356,6 +356,40 @@ func (this *IfElse) String () string {
 	return out
 }
 
+// Match is a control flow branching expression that executes one of several
+// case expressions depending on the input. It can be used to check the type of
+// a union value. Each case takes the form of a declaration, and an associated
+// expression. If the type of the union matches the type of the declaration in
+// the case, the expression is executed and the value of the union is made
+// available to it through the declaration. If the value of the match expression
+// is used, all possible types in the union must be accounted for. It may be
+// assigned to any type that satisfies the assignment rules of its first case.
+type Match struct {
+	// Syntax
+	Position errors.Position
+	Value    Expression
+	Cases    []*Case
+
+	// Semantics
+	Ty Type
+}
+func (*Match) expression(){}
+func (this *Match) Type () Type { return this.Ty }
+func (this *Match) HasExplicitType () bool {
+	if len(this.Cases) == 0 {
+		return true
+	} else {
+		return this.Cases[0].HasExplicitType()
+	}
+}
+func (this *Match) String () string {
+	out := fmt.Sprint("match ", this.Value, " on")
+	for _, cas := range this.Cases {
+		out += fmt.Sprint(" ", cas)
+	}
+	return out
+}
+
 // Loop is a control flow expression that repeats an expression until a break
 // statement is called from within it. The break statement must be given a value
 // if the value of the loop is used. Otherwise, it need not even have a break

entity/misc.go

@@ -74,6 +74,15 @@ func (this *Member) String () string {
 	return fmt.Sprint(this.Name, ":", this.Value)
 }
 
+// Case represents a match case.
+type Case struct {
+	Declaration Declaration
+	Expression
+}
+func (this *Case) String () string {
+	return fmt.Sprintf("| ", this.Declaration, this.Expression)
+}
+
 // Operator determines which operation to apply to a value in an operation
 // expression.
 type Operator int; const (