The Fortress Language Specification - CiteSeerX

Chapter 9
Traits
Traits are declared by trait declarations. Traits define new named types. A trait specifies a collection of methods
(described in Section 9.2). One trait can extend others, which means that it inherits the methods from those traits, and
that the type defined by that trait is a subtype of the types of traits it extends.
9.1 Trait Declarations
Syntax:
TraitDecl ::= TraitHeader (MdDecl | AbsFldDecl | PropertyDecl) ∗ end
TraitHeader ::= TraitMod ∗ trait Id [StaticParams] [Extends] [Excludes] [Comprises] [Where]
Extends ::= extends TraitTypes
Excludes ::= excludes TraitTypes
Comprises ::= comprises MayTraitTypes
TraitTypes ::= TraitType
| { TraitTypeList }
TraitTypeList ::= TraitType ( , TraitType) ∗
MayTraitTypes ::= {}
| TraitTypes
TraitType ::= DottedId [StaticArgList ]
| { TypeRef ↦→ TypeRef }
| 〈 TypeRef 〉
| TypeRef [ [ArraySize] ]
| TypeRef [ MatrixSize ]
ArraySize ::= Extent (, Extent) ∗
Extent ::= NatRef
| NatRef # NatRef
MatrixSize ::= NatRef (× NatRef ) +
Syntactically, a trait declaration starts with an optional sequence of modifiers followed by the special reserved word
trait , followed by the name of the trait, an optional sequence of static parameters (described in Chapter 11), an
optional set of extended traits, an optional set of excluded traits, an optional set of comprises on the trait, an optional
where clause (described in Section 11.6), a list of method declarations, abstract field declarations, and property
declarations (described in Section 19.6), and finally the special reserved word end .
Each of extends , excludes , and comprises clauses consist of the special reserved word extends , excludes ,
and comprises respectively followed by a set of trait references separated by commas and enclosed in braces ‘{’ and
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