0004: Trealla Prolog via C FFI Replacing Scryer-Prolog

Status#

Accepted

Date#

2026-04-19

Supersedes#

ADR-0002

Context#

ADR-0002 chose scryer-prolog (Rust) as the Prolog backend. After 9 days of production use, several pain points emerged:

• Build time: Rust compilation adds 60-90 seconds to clean builds and requires a separate toolchain install.
• GLIBC symbol patching: The objcopy workaround for Rust’s libc GLIBC version symbols is fragile and Linux-only.
• Binary size: The scryer-prolog Rust staticlib produces a ~50 MB binary.
• Contributor friction: Two toolchains (Zig + Rust) raise the onboarding bar.
• CI complexity: GitHub Actions workflows require Rust toolchain setup, cargo fmt, cargo clippy, and cross-compilation targets.

Trealla Prolog is a C99 ISO-compliant Prolog implementation that can be compiled directly by Zig’s built-in C compiler, eliminating all Rust dependencies.

Candidates#

Decision#

Replace scryer-prolog with Trealla Prolog, vendored as a git submodule at ffi/trealla/ and compiled via Zig’s built-in C compiler.

Specifically:

• C compatibility wrapper: A ffi/trealla-wrapper.c file exposes the same 9 prolog_* C-ABI functions that ffi.zig declares, translating between Trealla’s native API (pl_create, pl_eval, pl_consult, pl_destroy) and the existing interface. This means ffi.zig, engine.zig, all 24 tool handlers, and all 190+ tests need zero changes.
• JSON format translation: The C wrapper transforms Trealla’s per-solution output into the JSON array format [{bindings}, ...] that engine.zig:parseQueryResult() expects, maintaining full compatibility.
• Build integration: build.zig compiles Trealla’s C source files via addCSourceFiles(). No Cargo, no objcopy, no OpenSSL linkage.
• Submodule pinning: Trealla source is pinned to a specific commit via git submodule for reproducible builds.
• Compilation flags: Trealla is compiled with -DUSE_OPENSSL=0 -DUSE_FFI=0 for a minimal ISO Prolog core.

Consequences#

What becomes easier#

• Single toolchain: Only Zig >= 0.15.2 required to build. No Rust, no Cargo, no system OpenSSL.
• Faster builds: Clean build time reduced by ~50% (no Rust compilation step).
• Simpler CI: No Rust toolchain setup, no cargo fmt/cargo clippy steps, no cross-compilation target matrix.
• Smaller binary: Trealla is ~50K lines of C99 vs scryer-prolog’s ~200K lines of Rust.
• Cross-compilation: Zig’s built-in C compiler handles cross-compilation natively (-Dtarget=aarch64-linux-gnu).

What becomes harder#

• Trealla version updates: Must update git submodule and verify C wrapper compatibility.
• C wrapper maintenance: The compatibility wrapper (~200 lines) must be updated if Trealla’s API changes.

Constitution Compliance#