A directory cannot be a module on its own: there is no mod.mtl equivalent

GAP-MODULES-002

There is no mod.rs equivalent: a directory is not a module by itself, so name/mod.mtl has no meaning; the facade is a name.mtl file beside the name/ directory.

Lifecycle

accepted

Owner

language

Scope

reference/spec/modules.md#spec.modules.file-to-module-mapping.legality-1

Origin

maintainer note during the Atlas limitation review

Review

revisit if the name.mtl facade proves awkward for large module trees

Affects · linked records

spec.modules.file-to-module-mapping.legality-1language spec rule

RFC-0030rfc

gapKnown gaps ↗

Gap

Rust lets a directory be its own module through mod.rs (util/mod.rs declares util). Metel has no counterpart. :: maps directly to /, so a module path names exactly one file, and import root::util::answer; looks for util.mtl, never for util/mod.mtl. Reproduced against current develop:

// main.mtl
import root::util::answer;
fun main() { assert(answer() == 42); }

with the module in util/mod.mtl:

[P0001] parse error: cannot find module file for `util::answer`

The same program passes with the module in util.mtl. A directory module that needs a public facade is written as name.mtl alongside the name/ directory (parser.mtl re-exporting from parser/ast.mtl, parser/lexer.mtl); the two are distinct paths and coexist. The spec states this directly: there is no name/mod.mtl convention.

Impact

A module cannot keep its root file inside its own directory. Moving or renaming a module means moving a file and a directory that sit side by side, and the directory cannot be relocated as a self-contained unit the way a mod.rs directory can. Programs that group submodules under a directory must add the facade file next to it.

Affects

• spec.modules.file-to-module-mapping.legality-1
• RFC-0030

Resolution

Accepted as designed: RFC-0030 (module system redesign, implemented) chose ":: maps to /, no special directory module file", and the spec records that as Legality Rule file-to-module-mapping.legality-1, covered by a fixture. Reopen if directory-local module roots are wanted; that would be a new RFC.