The Fortress Language Specification - CiteSeerX

to the value of the condition expression. The two values are compared according to an optional operator specified.
If the operator is omitted, it defaults to = or ∈. If the condition expression has type Generator or if the guarding
expression does not, then the default operator is = ; otherwise, it is ∈. It is a static error if the specified operator is not
defined for these types or if the operator’s return type is not Boolean.
The right-hand side of the first matched clause (and only that clause) is evaluated. If no matched clause is found, a
MatchFailure exception is thrown. The right-hand side of each clause forms a block expression and has the various
properties of block expressions (described in Section 13.11). The optional else clause always matches. The value
of a case expression is the value of the right-hand side of the matched clause. The type of a case expression is the
union type of the types of all right-hand sides of the case clauses.
For example, the following case expression specifies the operator ∈:
case planet ∈of
{ Mercury, Venus, Earth, Mars } ⇒ “inner”
{ Jupiter, Saturn, Uranus, Neptune, Pluto } ⇒ “outer”
else ⇒ “remote”
end
but the following does not:
case 2 + 2 of
4 ⇒ “it really is 4”
5 : 7 ⇒ “we were wrong again”
end
13.21 Extremum Expressions
Syntax:
Flow ::= case (largest | smallest ) [Op] of (Expr ⇒ Expr + ) + end
An extremum expression uses the same syntax as a case expression (described in Section 13.20) except that the
special reserved word largest or smallest is used where a case expression would have a condition expression
and an extremum expression does not have an optional else clause.
All guarding expressions of an extremum expression is evaluated in parallel. See Section 4.4 for a discussion of
parallel evaluation. To find the largest (or smallest) quantity, the values of the guarding expressions are compared in
parallel according to an optional operator specified. If the operator is omitted, it defaults to CMP . The union of the
types of all the candidate expressions must be a subtype of TotalOrderOperatorsT, ,CMP for some T ,
< , ≤ , ≥ , and > , if a default operator is used (see Section 37.2 for details about the TotalOrderOperators trait).
If an explicit operator is used, the explicit operator replaces CMP in TotalOrderOperatorsT, ,CMP .
The right-hand side of the clause with the largest (smallest) guarding expression (and only that clause) is evaluated.
If more than one guarding expressions are tied for largest (smallest), the leftmost clause is evaluated. The righthand
side of each clause forms a block expression and has the various properties of block expressions (described in
Section 13.11). The value of an extremum expression is the value of the right-hand side of the matched clause. The
type of an extremum expression is the union type of the types of all right-hand sides of the clauses.
For example, the following code:
case largest of
1 mile ⇒ “miles are larger”
1 kilometer ⇒ “we were wrong again”
end
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