Glow Command line internals

A command line tool has a few parts:

  1. parser to parse command line options.
  2. handlers to deal with the parsed datastructure.

Where exactly is glow cli defined?

We can invoke it with ./glow, which invokes main.ss. In there, it runs the following:

(current-program "glow")
(initialize-glow-path!)
(define-multicall-main)

Let’s analyze this section by section:

(current-program "glow")
;; is evaluated to
((make-parameter []) "glow")

What is make-parameter?

If we look at Gambit - make-parameter, we know that the environment of execution has 2 parts:

  1. local
  2. global

We first access local environment, if a variable can’t be found, we then try global. A new cell is instantiated and bound to a new parameter object. The content of this cell is []. Whenever an argument, in this case "glow", is passed to the parameter object the contents get updated. If no arguments are passed, this just returns the cell contents.

Example

> (define radix (make-parameter 10))
> (radix)
10
> (radix 2)
> (radix)
2

The top-level module for `glow-cli` is located in `cli/interaction.ss`

What is `define-multicall-main`?

It is called `make-parameter`, because the associated variable is dynamically bound!

Reference: https://srfi.schemers.org/srfi-39/srfi-39.html

Also note:
This is not thread safe for gambit according to be above srfi? 

Q: What's the point though?
Can't we just do `set!` against a variable, doesn't that accomplish the same thing? 
A: This is for `dynamic` variables.


### define-multicall-main

First let's decrypt what is a <d223d1a2>.
Next we can see we define a `main` entrypoint,
which is multicall, `gxi` uses this to know the program entrypoint.
Based on that this uses `call-entrypoint` which calls `call-entrypoint/internal`.
This then gets the entrypoints by the `command`, i.e. the first argument.
It then calls that command.

Next, we observe in `cli/interaction.ss`, and other places we use
`define-entry-point` to start an interaction.

## What do we use to construct our CLI parser?

We use `gerbil-poo/cli` which uses `getopt`.

## How does `gerbil-poo/cli` work?

Its interface is located in `gerbil-poo/cli`.
It uses `make-options`