clarify that the Nix language is not called "Nix expressions"

[mschwaig]

Motivation

I had a student come by with a first draft of his thesis, and in it he referred to the Nix language as Nix expressions.

Context

The student showed me how he arrived at that conclusion by reading the introduction of the Nix manual.
If you're not aware of expression as an FP term and don't have prior knowledge of the fact that Nix likes to call every piece of Nix Nix, then the introduction can in fact be read like that.

IMO by relying on the term expressions so much this early the manual uses language to dance around these naming issues, which makes things actually more difficult to understand than if our docs would address that issue head on, which is what I tried to do in this PR.

Alternatives to consider:

• Going through the manual and qualifying every instance of the term nix Nix with something like Language or Package Manager.
• Naming some of these things something else.

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[edolstra]

I don't think this is more readable, since it immediately raises the question "what are expressions?" or "what expressions?"

Might be better to say something like

In Nix, the package build graph is described using a simple functional language. Programs in this language are called Nix expressions.

[mschwaig]

Ok, so programs written in Nix are called Nix expressions. I though expression really just meant the functional programming / programming language term, expression. Thanks!

[mschwaig]

I will think about this a bit, because I find it a bit surprising that the programs are called expressions.

[edolstra]

The reason is that a .nix file is an expression at top-level. Unlike most languages there is no top-level module syntax (e.g. a list of declarations).

[edolstra]

While I would love to get rid of the term "Nix expression" and just call it the Nix language, the CLI today does refer to "Nix expressions" everywhere. So getting rid of that terminology is a much bigger job than 2 lines in the intro.

[lf-]

Lix's style guide calls the language "Nix language" and I see no harm in being consistent with it.

I would write this sentence as "Packages are built from expressions in the Nix language, which is a simple functional language.

The next sentence in here is also .. kind of weird. I guess it's trying to say that "a derivation is fully described by a single nix expression", but adding emphasis there is sort of confusing as well, since it makes it sound like nix expression is a glossary term. Maybe it is? I'm not familiar with the modern style guide efforts in the orig-Nix project.