requires — Gating Capabilities

Three requires in sequence; the compiler validates them before any fn.

// Feature: `enable` and `requires` directives
// Syntax: `enable <feature>;` / `requires <capability>;`
// When to use: gate a module on backend capabilities (FS, signals,
//   threads, GPU, ...) or platform (POSIX vs Windows). Build fails
//   early with a clear message instead of panicking at runtime.
//
// See `specs/enable-requires-directives.md` for the full specification.

requires backend.vm
requires feature.fs
requires version >= 0.1

// `enable` activates optional language features. Unknown features emit
// a warning (forward-compat); enabling a feature unsupported on the
// current backend is a hard error.
//
//   enable simd          // available only on backend.native / .wasm
//   enable signals       // available on backend.vm / .native
//
// Diretivas devem aparecer no topo do arquivo, antes de `use`, `fn`,
// `struct`, etc. Diretiva após declaração → E_DirectiveNotAtTop.

fn main() {
  print("directives validated at build time; nothing to do in runtime")
}
// Run:
//   zolo run 01-enable-requires.zolo
//   → directives validated at build time; nothing to do in runtime
//
// Try changing `requires backend.vm` to `requires backend.native` —
// the build will now fail with E_CapabilityMissing pointing at the
// directive, before any of your code runs.

requires feature.fs + requires feature.process guarantee both before execution.

// Feature: gating a module on multiple capabilities + a minimum
// language version.
//
// `requires` can be stacked — every clause must hold or the build
// aborts with `error[D0002]`. Common gates:
//   - `requires backend.{vm,native,wasm}`
//   - `requires platform.{posix,linux,macos,windows,wasm}`
//   - `requires feature.{fs,net,process,signals,threads,gpu,...}`
//   - `requires version >= 0.1`        // monotonic, never expires
//
// See specs/enable-requires-directives.md for the full capability table.

requires version >= 0.1
requires feature.fs
requires feature.process

use std::fs
use std::process

fn main() {
  // Reaching this line guarantees fs + process are available on the
  // active target — no runtime fallback needed.
  print("pid: {process.pid()}")
  let cwd_ok = fs.exists(".")
  print("cwd writable: {cwd_ok}")
}

// Try editing the file to add `requires backend.wasm` — building for
// the VM target should now fail with a clear diagnostic instead of
// crashing at runtime.