Equations

lhs.syn = ... definition of synthesized attribute;
rhs.inh = ... definition of inherited attribute;
local.inh = ... similar;

Attribute definitions specify the values of attributes as some expression over the values of attributes on neighboring nodes in the syntax tree. In a production, definitions for synthesized attributes of the left-hand side nonterminal and for inherited attributes on the right-hand side nonterminals (as well as locals) may be specified. These have the form

<short-name> . <name> = <expression> ;

Consider the following definitions of synthesized attributes pp and value that decorate the Expr nonterminal in the grammar tutorials:dc. These are located in the AbstractSyntax.sv file in that grammar.

abstract production add
sum::Expr ::= l::Expr r::Expr
{
 sum.pp = "(" ++ l.pp ++ " + " ++ r.pp ++ ")";
 sum.value = l.value + r.value ;
}

Here we see that the string-valued pp attribute on the sum node is computed by concatenating string literals and the value of the pp attribute on the right hand side nonterminals. Similarly, for value, which is computed by adding the values of that same attribute on the child nodes.

As an example of inherited attributes definitions consider the following example from the file Stmt.sv in the tutorials:simple:abstractsyntax grammar.

s::Stmt ::= s1::Stmt s2::Stmt 
{
  s1.env = s.env ; 
  s2.env = appendEnv(s1.defs, s.env) ;
  s.defs = appendEnv (s1.defs, s2.defs) ;
}

This production defines two statements (which may be compound statements) in sequence. Here we see that the environment attribute env on child s1 is copied down to this child from its parent s. For s2, the environment is computed from the definitions on s1 and the environment on s. If s1 is a variable declaration, then that declaration information is stored in s1.defs and will thus now be available to s2. This follows the normal scoping rules found in most imperative languages.

Silver supports many types and operations over those types to write expressions:

• Numeric expressions
• Boolean expressions,
• comparison expressions
• string operations and built-in functions
• other built-in functions

are discussed within this Wiki.