The Fortress Language Specification - CiteSeerX

Chapter 17
Conversions and Coercions
Fortress provides a mechanism, called coercion, to allow a value of one type to be automatically converted to a value of
another type. Programmers can define coercions (described in Section 17.2) and coercions may change the set of applicable
declarations for a call as described in Section 17.4. If multiple coercions can be applied to a particular functional
call then the most appropriate coercion is chosen statically. Restrictions on coercion declarations (described in Section
17.6) guarantee that the static resolution of coercion (described in Section 17.5) is well-defined. Fortress provides
implicit coercions for tuple types and arrow types (described in Section 17.7). Fortress also provides an additional
feature of coercion that allows “widest-need” evaluation of numerical expressions (described in Section 17.8).
17.1 Principles of Coercion
In certain situations it is convenient to be able to use a value of one type T as if it were a value of another type U
even though T is not a subtype of U . For example, it is convenient to be able to use the integer-valued expression
2 in a floating-point expression even though its type is not a subtype of any floating-point type. Fortress supports the
automatic conversion of integer values to floating-point values (the technical term for this kind of automatic conversion
is coercion). In this way one can write (x + 1)/2 rather than (x + 1.0)/2.0 , for example. Such coercion applies
generally to function and method (collectively called as functional) calls as well as to operators; one can write ln 2
rather than ln 2.0 , or arctan(1, 1) rather than arctan(1.0, 1.0) , for example.
One way to think about coercion is that if type T can be coerced to type U , then a value of type T can be used to
“stand in” for a value of type U in a functional call. This is different from actually being of type U ; it means only
that, given any value of type T , an appropriate value of type U can be computed to substitute for it. Also, coercion
occurs only when the declared type of the corresponding parameter in the functional declaration is exactly the type U
being coerced to (in whose declaration the coercion is defined) not if it is a supertype of U . Note, however, that if
type T can be coerced to type U then any subtype of T can also be coerced to type U .
Coercion from T to U may also occur in variable definitions and assignment expressions when the declared type of
the left-hand side is U and the type of the expression on the right-hand side is a subtype of T .
Coercion is not automatically chained in Fortress, unlike some other programming languages: even if type T can be
coerced to type U and type U can be coerced to type V , it is not necessarily the case that type T can be coerced
to type V . Type T can be coerced to type V only if trait V itself contains an appropriate coercion declaration to
handle that particular type coercion.
Example 1: For any floating-point parameter, a decimal integer literal argument may be used.
Example 2: For any floating-point parameter, a floating-point argument of a shorter format may be used.
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