Type Checking¶

Overview¶

TypeCheckEASTVisitor traverses the Enriched AST and verifies that every expression has a well-formed type. It returns a TypeNode for each node, propagating types bottom-up through the tree.

Subtype Relations¶

Type compatibility is determined by TypeRels.isSubtype(a, b):

BoolTypeNode is a subtype of IntTypeNode (booleans can be used as integers).
A class type C is a subtype of D if C extends D (directly or transitively).
EmptyTypeNode (null) is a subtype of any RefTypeNode.

Key Rules¶

Construct	Type Rule
`IntNode`	`IntTypeNode`
`BoolNode`	`BoolTypeNode`
`PlusNode`, `MinusNode`, `TimesNode`, `DivNode`	Both operands must be `int`; result is `IntTypeNode`
`AndNode`, `OrNode`	Both operands must be `bool`; result is `BoolTypeNode`
`NotNode`	Operand must be `bool`; result is `BoolTypeNode`
`EqualNode`	Operands must share a common subtype
`GreaterEqualNode`, `LessEqualNode`	Both operands must be `int`; result is `BoolTypeNode`
`IfNode`	Condition must be `bool`; result is the lowest common ancestor of the two branch types
`CallNode`	Argument types must be subtypes of the declared parameter types; result is the function's return type
`NewNode`	Argument types must match class field types; result is `RefTypeNode(classID)`
`ClassCallNode`	Like `CallNode` but resolves through the virtual table
`VarNode`	Declared type must be a supertype of the initialiser type
`FunNode` / `MethodNode`	Body type must be a subtype of the declared return type

Lowest Common Ancestor¶

For if / then / else, the result type is the lowest common ancestor (LCA) of the two branch types in the subtype hierarchy. For example, if one branch has type C and another has type D where both extend Base, the result type is Base.

Error Reporting¶

Type errors are printed to standard error and counted. Compilation continues after a type error so that further errors can be reported in the same run.