Nix language tutorial: restructure introduction

[fricklerhandwerk]

this collapses the "don't panic" section into a motivating introduction and gives
the document a more distinctive name.

this also addresses feedback stating that having "basics" in the title
may be misleading about the effort required to work through the
tutorial.

this also fixes a long-standing gap of not mentioning function arguments as another form of name assignment.

@NixOS/documentation-team

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[roberth]

What is the process here? Are you extending the introduction because it's too short? Is the desired length of an introduction documented somewhere?

[fricklerhandwerk]

No, the intent is to focus the introduction on the motivation for the particular approach. I was thinking about cutting down on the elements of programming, but didn't find a good way so there they still are. How would you illustrate to someone not necessarily familiar with functional programming that a language is small and has few fundamental moving parts?

[roberth]

How would you illustrate to someone not necessarily familiar with functional programming that a language is small and has few fundamental moving parts?

I'd sneak in the three elements of lambda calculus (variable reference, lambda abstraction, application):

• functions: you can describe complex behavior by introducing a name for something you don't have
• function application: you can use such behaviors by filling in the thing that was missing, as many times as you need
• names: you can use a name to refer to something you've introduced with a function, or what's made available to you by the programming environment

So this de-emphasizes the role of "primitive values" compared to The Elements Of Programming you referenced.
Not coincidentally that's the purpose of lambda calculus: a simplified model of computation, and indeed all data can be expressed using only a function primitive. (e.g. consider true = a: b: a; false = a: b: b; or the more general encoding schemes that exist for "usable" data types)

Nix is a functional language, so maybe this is a useful model?
When you have the lambda calculus part down, and the sugar like let, inherit, and the unrelated monster with, you're going to be thinking about exactly all the other things that make up your problem domain, which is of course the bread and butter of what we're documenting. So that seems like a sensible boundary for dividing up the material "reference style", but for a learning experience, I guess we'll want to return to this subject to "exercise" the lambda calculus thinking. It's not just a thing you use but a cognitive skill.

[fricklerhandwerk]

• @proofconstruction, @renoire: have some concerns about inclusion of the time expected section
  • should add some introduction how we came up with these numbers
• @fricklerhandwerk: should move the time estimate after the prerequisites

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-06-26-documentation-team-meeting-notes-58/29666/1

[fricklerhandwerk]

I decided against renaming the article. The title should stay brief and unique, and it is. The content changes are already an improvement.