DOC: replace dream with sexp in first program

[heathhenley]

Related to #1962

What does everyone think about something like this? I tested on windows and it works, the example is just pulled from Yojson's readme, but we could make it 'learning ocaml' specific if you prefer.

Two things:

• after installing the package using opam, I had to run the opam env update described here: https://ocaml.org/docs/installing-ocaml#2-create-an-opam-switch (not creating the switch just update the env). I'm wondering if that should be mentioned in the tutorial? Opam does tell you to run eval $(opam env) but that only helps if you're running linux based...
• is there a better way to describe the type that Yojson returns that is more instructive for beginners?

[F-Loyer]

I guess I was too fast when suggesting yojson. It is small, not system-dependant, useful... but it uses polymorphic variants which I don't think they are introduced in the learning/language section.

sexplib belongs to the same kind of library, but Sexp instead of JSON. The result type doesn't use polymorphic variants, but simple ADT.

[heathhenley]

Ah, no worries I'll give that one a shot instead. I went with JSON because it was more familiar, now I get to learn what S expression is 🤣

[F-Loyer]

"is there a better way to describe the type that Yojson returns that is more instructive "

Some programs need a configuration file. Then if we read it, it can be convenient to parse it, especially if the config is complex (variable assigned to list...). Then a program which extract a value could demonstrate an interest.

JSON and Sexp both permit this kind of configuration.

Note Polymorphic variants ADT are very close to the usual ADT... Perhaps we could use them.

(You already have typed a Sexp... dune file are like this).

[heathhenley]

Here's another option based on the suggestion of Sexplib - let me know what you all think

[christinerose]

Suggested change

	The string you entered representing a valid S-expression is parsed into `List`of `List`s of `strings` so that you can use it in your program. Refer to the [Sexplib documentation](https://github.com/janestreet/sexplib) for more information.
	The string you entered representing a valid S-expression is parsed into `List` of `List`s of `strings` so that you can use it in your program. Refer to the [Sexplib documentation](https://github.com/janestreet/sexplib) for more information.

[heathhenley]

Thanks for the corrections! A pushed a change to the type definition thanks to the clarification by @F-Loyer here so this suggestion looks outdated. Though based on their correction (that I just noticed) this probably needs another tweak to be more correct...

[F-Loyer]

Beware, I was wrong in my first comment (I have edited). It is not a list of atom (string) or sexp, but an atom (string) or a list of s-exp.

[christinerose]

A few suggestions and typos.

[christinerose]

I see several of my suggestions are now outdated, so I'll go through it again when the content is set. 🙂

[heathhenley]

I updated the type def - I think it's ready for a re-review / input whether it's a good enough example to switch for dream.

[christinerose]

Looks great!

[cuihtlauac]

This looks good to me, only small changes suggested.

[cuihtlauac]

@jonahbeckford: What's your view on this PR? It should be good for Windows?

[jonahbeckford]

It works on Windows, yes. (I can't think of a more portable example, although I'm not enthused about the example itself. Better than the alternate so LGTM)