Semantic Analysis

Semantic analysis is the phase of compilation in which a program's meaning is checked against the rules of the language, after its syntax has been recognized and represented as an abstract syntax tree. Where parsing asks whether the program is grammatically well-formed, semantic analysis asks whether it is meaningful: whether variables are declared before use, whether types are compatible, whether control paths return values, whether access modifiers are respected.

The boundary between syntax and semantics is itself contested. Some properties — like type consistency — are treated as syntactic in some languages (statically typed) and as dynamic runtime checks in others. The choice is not technical but philosophical: it encodes a commitment about which errors are discoverable by analysis of the program text alone and which require knowledge of the execution context.

Semantic analysis operates by traversing the AST and maintaining a symbol table — a mapping from identifiers to their declarations, types, and scopes. The analysis is often context-sensitive: the meaning of an identifier depends on the nested scopes in which it appears, and the semantic analyzer must track scope entry and exit as it descends and ascends the tree. This interplay between tree structure and contextual state makes semantic analysis one of the most subtle phases of compilation, and the one where language design decisions have their most visible consequences.