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C++ Compare Evaluator for Shimmy WASM

This example is a minimal, self-contained C++ evaluator that compiles to a WebAssembly module and runs through Shimmy's opt-in WASM backend. It is not a port of an existing Lambda Feedback repository; it is a small Go/C++-style artifact example for validating the generic WASM execution path.

It intentionally mirrors the shape of a simple Lambda Feedback evaluator:

  • input: response, answer, and optional feedback strings in params
  • output: { "command": "eval", "result": { "is_correct", "feedback" } }

The evaluator also reports guest_invocation_count and snapshot_isolation_ok so the demo and integration test can prove that Shimmy reuses a warm WASM instance while restoring guest memory after each request.

Build

The reference environment for this example is Linux, or a Linux container, with Zig installed. The same command also works on macOS when Zig is installed; the point is to rely on an explicit WASM-capable toolchain rather than the host's default C++ compiler.

zig c++ \
  -target wasm32-freestanding \
  -Oz \
  -nostdlib \
  -fno-exceptions \
  -fno-rtti \
  -Wl,--no-entry \
  -Wl,--export=alloc \
  -Wl,--export=evaluate \
  -Wl,--export-memory \
  -Wl,--initial-memory=2097152 \
  -o eval.wasm \
  evaluator.cpp

The source avoids libc/libc++ and implements only the small amount of JSON handling needed for this evaluator, so it can be built as a small freestanding WebAssembly module. Real C++ evaluators can use a richer build setup, but still need to expose the same Shimmy WASM ABI:

memory
alloc(size: i32) -> i32
evaluate(req_ptr: i32, req_len: i32) -> i32

evaluate returns a pointer to [uint32 little-endian response_len][response JSON bytes].

Run the end-to-end demo

From the repository root:

./scripts/demo-cpp-wasm.sh

The script builds Shimmy, compiles this evaluator to eval.wasm, starts Shimmy with FUNCTION_INTERFACE=wasm, sends two HTTP requests, and asserts that both requests see guest_invocation_count == 1.

The Go test suite also compiles this example when zig is available:

go test ./internal/execution/wasm -run TestCppCompareExample_CompilesAndRunsThroughDispatcher -v