Adding New Extern Providers

In Plywood, extern providers are defined by adding a special C++ function to a file with the suffix .modules.cpp somewhere in a repo's directory tree. This function is called an extern function and must be preceded by a line comment of the form:

// [ply extern="<extern-name>" provider="<provider-name>"]

<extern-name> can be prefixed with the name of a repo. If <extern-name> refers to the current repo, a new extern with that name is implicitly created and added to the current repo. If <extern-name> was already defined by another repo that the current repo depends on, a new provider is added to the set of available providers for that extern.

For example, one of Plywood's built-in extern providers defined in repos/plywood/src/web/web.modules.cpp uses the extern name "libsass" and provider name "prebuilt":

// [ply extern="libsass" provider="prebuilt"]
ExternResult extern_libsass_prebuilt(ExternCommand cmd, ExternProviderArgs* args) {
    // Toolchain filters
    if (args->toolchain->targetPlatform.name != "windows") {
        return {ExternResult::UnsupportedToolchain, "Target platform must be 'windows'"};
    }
    if (find<StringView>({"x86", "x64"}, args->toolchain->arch) < 0) {
        return {ExternResult::UnsupportedToolchain, "Target arch must be 'x86' or 'x64'"};
    }
    if (args->providerArgs) {
        return {ExternResult::BadArgs, ""};
    }
    ...

The name of the function itself (in this case, extern_libsass_prebuilt) isn't really important, except that it must be unique across all .modules.cpp files in the current repo. By convention, the function name usually starts with extern_.

The set of valid operations that can be performed by an extern provider is not yet documented. For the time being, you can study existing examples in the plywood repo.