The Fortress Language Specification - CiteSeerX

esult
end
where result is a fresh variable. A reduction expression without a generator list:
∑ g
is equivalent to the following:
∑ [x ← g]x
Note that reduction expressions without generator lists can be used to conveniently sum any aggregate expression
(described in Section 13.28), since every aggregate expression is a generator.
13.19 If Expressions
Syntax:
Flow ::= if Expr then Expr + (elif Expr then Expr + ) ∗ [Else] end
| (if Expr then Expr + (elif Expr then Expr + ) ∗ Else [end ])
Else ::= else Expr +
An if expression consists of the special reserved word if followed by a condition expression of type Boolean,
followed by the special reserved word then , a sequence of expressions, an optional sequence of elif clauses (each
consisting of the special reserved word elif followed by a condition expression, the special reserved word then ,
and a sequence of expressions), an optional else clause (consisting of the special reserved word else followed by a
sequence of expressions), and finally the special reserved word end . Each clause forms a block expression and has the
various properties of block expressions (described in Section 13.11). An if expression first evaluates its condition
expression. If the condition expression evaluates to true, the then clause is evaluated. Otherwise, the next to the
then clause is evaluated. An elif clause evaluates its condition expression first and proceeds similarly to an if
expression. The type of an if expression is the union of the types of all right-hand sides of the clauses. If there is no
else clause in an if expression, then the last expression in every clause must evaluate to () . The special reserved
word end may be elided if the if expression is immediately enclosed by parentheses. In such a case, an else clause
is required.
For example,
if x ∈ {0, 1, 2} then 0
elif x ∈ {3, 4, 5} then 3
else 6 end
13.20 Case Expressions
Syntax:
Flow ::= case Expr [Op] of (Expr ⇒ Expr + ) + [Else] end
A case expression begins with the special reserved word case followed by a condition expression, followed by an
optional operator, the special reserved word of , a sequence of case clauses (each consisting of a guarding expression
followed by the token ⇒, followed by a sequence of expressions), an optional else clause (consisting of the special
reserved word else followed by a sequence of expressions), and finally the special reserved word end .
A case expression evaluates its condition expression and checks each case clause to determine which case clause
matches. To find a matched case clause, the guarding expression of each case clause is evaluated in order and compared
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