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Oil Modules Safely Separate a Program Into Files

Oil has a minimal module system that is shell-like.

• "Modules" is actually a misnomer because they are NOT "modular". Procs are modular. But we use the term since "module" is sometimes associated with file".
• In contrast to other features, it's very different than Python or JavaScript modules, which have multiple global namespaces.

The only goal is a little more safety.

An Example

Library file. Top level has module, source, const, and proc.

# lib-foo.oil (no shebang line necessary)

module lib-foo.oil || return 0  # module named after file
source $_this_dir/lib-other

const G_foo = {myvar: 42}

proc foo-proc {
  echo 'hi from foo-proc'
}

# no main function

Executable file. Top level the same 4, plus oil-main at the bottom.

#!/usr/bin/env ysh

# deploy.ysh: Deploy C++ program to a server
module main || return 0  # executable programs use 'main' guard

source $_this_dir/lib-foo.oil
source $_this_dir/lib-bar.oil

const DEST_HOST = 'example.com'
const DEST_USER = 'homer'

proc .private-p {
  echo "I don't want oil-main to find this"
}

proc _p {
  .private-p  # direct function call
  foo-proc
  echo hi
}

proc p {
  sudo $0 _p @ARGV  # argv dispatch pattern
}

oil-main  # dispatch to arguments in this module, except ones beginning with .

Usage Guidelines

• Distinguish between .oil files that are executable programs, and those that are libraries
  • A lib- prefix or a lib/ dir can make sense, but isn't required
• Every file needs a module guard
• Use oil-main
  • Optional "hide" symbols with .

Other:

• source must only be used at the top level.
• When using modules, it's considered bad style to put code at the top level.
  • Either ALL code is at the top level in short script, or NONE of it is.
  • See the doc on variables.

Recap: Shell Has Separate Namespaces for Functions and Variables

TODO:

• Proc namespace
• Var namespace (a stack)

The source just concatenates both.

This is like a Lisp 2.

Oil doesn't deviate from this! It builds some things on top.

TODO: See Interpreter state / data model.

Mechanisms

Guarded Execution with module

• Use either main or mylib.oil

$_this_dir For Imports Relative To the Current File

• This lets you move things around, version them, etc.

The oil-main builtin dispatches to procs

The $0 dispatch pattern.

Shell Options redefine_{proc,module} Expose Name Conflicts

In batch mode, you'll get errors.

But you can iteratively test in interactive mode.

source mymodule.oil  # 'module' guard will be bypassed in interactive mode

Bundling

Oil Source Files

TODO / help wanted: Pea.

It's nice that we have this "sequential" or concatenative property of code! Multiple "modules" can go in the same file.

Naming convention: pkg-foo.oil ? I don't really think we should have packages though?

With The Oil Interpreter

Appendix: Related Documents

• Variables and Namespaces
• QSN