The Fortress Language Specification - CiteSeerX

Example 3: For any floating-point interval parameter, a non-interval floating-point argument (of the same or shorter
floating-point format) may be used.
17.2 Coercion Declarations
Syntax:
Coercion ::= coercion [StaticParams](Id IsType)CoercionClauses = Expr
CoercionClauses ::= [Throws] [CoercionWhere] [Contract]
CoercionWhere ::= where { CoercionWhereClauseList }
CoercionWhereClauseList ::= CoercionWhereClause ( , CoercionWhereClause) ∗
CoercionWhereClause ::= WhereClause
| TypeRef widens or coerces TypeRef
To declare that trait U allows a coercion from type T , the declaration of trait U must provide a coercion declaration
whose parameter type is T . Coercion declarations are like functional declarations, except that coercion is actually
a special reserved word, a coercion declaration may have special where -clause constraints (described below), and it is
not permitted to specify a return type, because the return type must be the very trait in whose declaration the coercion
declaration appears. The coercion body is required; there is no such thing as an abstract coercion declaration.
Coercions may have static parameters (described in Chapter 11) and a where clause (described in Section 11.6), just
like functionals. The where clause may contain one of the following special constraints:
Type 1 coerces Type 2
Type 1 widens Type 2
Type 1 widens or coerces Type 2
The first is true if trait Type 1 has a coercion from Type 2 . The second is true if trait Type 1 has a widening coercion
(described in Section 17.8) from Type 2 . The third may be used only in a coercion with the “widening ” special
reserved word; it is true if Type 1 has a coercion from Type 2 , and furthermore the coercion declaration to which the
where clause belongs is widening only if Type 1 has a widening coercion from Type 2 . For example:
trait VectorT extends Number
widening coercion U extends Number(x :VectorU)
where { T widens or coerces U } = . . .
end
Coercions, unlike methods, are not inherited. If trait V extends trait U , and trait U has a coercion from type T , then
V does not thereby have a coercion from type T . (It may, however, have its own coercion from type T , separately
defined within the body of V .) For example, given the following declarations,
trait A
coercion (b :B) = . . .
end
object B end
trait C extends A end
f(c :C) = 5
a call to f(B) is considered a static error because trait C does not inherit a coercion from type B .
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