Detect Use-Before-Declaration and Undefined Variables

Names Must Exist Before Use

TypeScript reports used before its declaration; C warns on implicit declarations. OpenSSL once shipped bugs from calling functions that were never declared. The fix is a two-pass or ordered walk: process declarations before uses in each scope, or reject forward references where the language forbids them.

Detection Strategy

Maintain a set of declared names per scope. On declaration, insert with line number. On use, verify the name exists and that the declaration line precedes the use line. Distinguish undefined (never declared) from use-before-decl (declared later).

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• Track declaration line vs use line
• Block scope: inner declarations visible only after their line
• Globals may allow forward declarations depending on language
• Typos in names look like undefined variables

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will detect variables used before declaration and references to undefined names. This exercise asks you to walk simple statements, maintain a declaration table, and emit precise errors with line numbers.