• Conceptually, attributes are functions on entire ASTs
• attr: AST x Node -> AVal
• Needs restrictions for efficient evaluation
• In practice, attributes are evaluated by
• Scanner: attached to tokens (e.g. source locations) - too restrictive
• Parser: integrated with AST construction - common use
• Attribute Evaluator: extra AST traversals - Common in later phases(e.g. code generation)

• An attribute at a node can only be computed form attributes at parents, siblings and children
• Synthesized attributes are computed from children
• Inherited attributes are computed from parents and siblings

• Attribute evaluation rules can be added to parse rules
• decl returns [String type, List vars]:
• t = type ids+=ID (',' ids+=ID)*
• {$type = $t.text; $vars = $ids};

Synthesized attribute for non-terminal A at node N 

• Defined by evaluation rule associated with production at N
• Production must have A as its head
• Defined in terms of N's children and N itself

Inherited attribute for non-terminal B at a node N

• Defined by evaluation rule associated with production at parent of N
• Production must have B in its body
• Defined in terms of N's parents and siblings, and N itself

• We can limit the use Synthesized an  inherited attributes to guarantee that the attribute evaluation is cycle-free:
• A grammar is S-attributed if every attribute is synthesized
• A grammar is L-attributed if every attribute is 
• Synthesized or
• Inherited, with the restriction that the evaluation rule uses only:
• Attributes inherited from the parent or
• Attributes (Synthesized or inherited) form left siblings

• S-attributed grammars are well suited for LR-parsing (bottom-up):
• When applying a reduce operation, all values are available to calculate the attributes of the parent
• L-attributed grammars are well-suited for LL-parsing (top-down, left-to-right):
• When calling a parse rule function, all attribute values are available to calculate the attributes of the non-terminal
• L-attribute grammars can be used for LR parsing
• inheritance from above is tricky