Implement Wasm Validation: Type-Check a Module Before Execution

Validating Wasm Modules

Browsers refuse to instantiate invalid modules. Validation is a static type and stack check over every function body before execution, catching bad stacks and type mismatches early.

Type and Stack Checking

The validator tracks:

• Operand stack depth and types at each instruction
• Function types against the type section
• Branch targets with matching stack polymorphism rules

Rules mirror the spec: \`i32.add\` requires two \`i32\` values; \`call\` must target a function with compatible params; \`end\` must leave the stack matching the enclosing block signature.

For example, \`i32.add\` with only one \`i32\` on the stack is a validation error, not a runtime surprise.

\`\`\` error: type mismatch at i32.add stack: [i32] (expected [i32 i32]) \`\`\`

Design Notes

Validate before execute on every untrusted module. Browsers depend on this pass for safety; your interpreter should refuse modules that fail stack typing rather than corrupting memory at runtime.

For This Exercise

This exercise asks you to implement a validator for a Wasm subset before your interpreter runs code. You will type-check function bodies and reject modules with impossible stack states.

You will use the same mental model here when reading production interpreter source later in the track. Sketch one concrete input on paper, predict the outcome, then confirm with code. That discipline catches logic errors early and makes debugging far faster when you extend the implementation in follow-on tasks.