The Fortress Language Specification - CiteSeerX

Chapter 36
Support for Domain-Specific Languages
To support syntax for domain-specific languages, and to allow the Fortress language to grow with time, programmers
can extend the basic syntax of Fortress in their programs. Such extensions are possible through the use of syntax
expanders. Syntax expanders must be defined in the top level of a simple component.
36.1 Definitions of Syntax Expanders
Syntax:
Decl ::= ExternalSyntax
ExternalSyntax ::= syntax OpenExpander Id CloseExpander = Expr
OpenExpander ::= Id | LeftEncloser
CloseExpander ::= Id | RightEncloser | end
The definition of a syntax expander starts with the special reserved word syntax , followed by an opening delimiter,
followed by a contents parameter, followed by a terminating delimiter, followed by an = , and a subexpression. The
opening delimiter must be either an identifier or the opening member of an enclosing operator. The contents parameter
must be an identifier (see Section 5.15). The terminating delimiter must be either an identifier, the terminating member
of an enclosing operator, or the special reserved word end . If either the opening delimiter or the closing delimiter is
part of an enclosing operator, the opening and closing delimiters must both be parts of enclosing operators, and they
must match, or it is a static error. Because delimiters conceptually delimit blocks, just as do and end delimit blocks,
delimiters of syntax expanders are rendered as special reserved words. The subexpression of a syntax expander has
type Fortress.Ast.SyntaxTree . This SyntaxTree must be that of a Fortress expression. Here is an example:
syntaxsql e end = parseSQL(e)
where parseSQL is a function that takes an argument of type Fortress.Lang.SourceAssembly (a sequence of
Unicode characters and abstract syntax trees), interprets it as an SQL query, and returns an expression with type
SyntaxTree consisting of constructor calls to SQL syntax nodes (defined in some SQL library).
At a use site, all characters between the opening delimiter and the terminating delimiter are turned into a SourceAssembly
(see Section 36.4 for a more detailed description of how this conversion is achieved). The resulting SourceAssembly
is bound to the contents parameter of the syntax expander. The use site is then expanded by evaluating the body of the
expander. Every use site of a syntax expander must occur in an expression context, or it is a static error.
For example, we could define parseSQL so that a use site such as:
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