docs(tvix): Begin specification of the Nix language

This is only the introductory text, without any of the components
specified so far.

Change-Id: I6cd8eff6ec2daf14cc2420ee0a8e8983bfaa57d9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3052
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/docs/language-spec.md

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+---
+title: "Specification of the Nix language"
+numbersections: true
+author:
+- tazjin
+email:
+- tazjin@tvl.su
+lang: en-GB
+---
+
+The Nix Language
+================
+
+WARNING: This document is a work in progress. Please keep an eye on
+[`topic:nix-spec`](https://cl.tvl.fyi/q/topic:nix-spec) for ongoing
+CLs.
+
+Nix is a general-purpose, functional programming language which this
+document aims to describe.
+
+## Background
+
+Nix was designed and implemented as part of the [Nix package
+manager](https://nixos.org/nix). It is primarily used for generating
+so-called [*derivations*](#derivations), which are data structures
+describing how to build a package.
+
+The language has been described in the
+[thesis](https://edolstra.github.io/pubs/phd-thesis.pdf) introducing
+the package manager, but only on a high-level. At the time of writing,
+Nix is informally specified (via its only complete implementation in
+the package manager) and there is no complete overview over its -
+sometimes surprising - semantics.
+
+The primary project written in Nix is
+[nixpkgs](https://github.com/NixOS/nixpkgs/). Uncertainties in the
+process of writing this specification are resolved by investigating
+patterns in nixpkgs, which we consider canonical. The code in nixpkgs
+uses a reasonable subset of the features exposed by the current
+implementation, some of which are *accidental*, and is thus more
+useful for specifying how the language should work.
+
+## Introduction to Nix
+
+Nix is a general-purpose, partially lazy, functional programming
+language which provides higher-order functions, type reflection,
+primitive data types such as integers, strings and floats, and
+compound data structures such as lists and attribute sets.
+
+Nix has syntactic sugar for common operations, such as those for
+attribute sets, and also provides a wide range of built-in functions
+which have organically accumulated over time.
+
+Nix has a variety of legacy features that are not in practical use,
+but are documented in sections of this specification for the sake of
+completeness.
+
+This document describes the syntax and abstract semantics of the Nix
+language, but leaves out implementation details about how Nix can be
+interpreted/compiled/analysed etc.
+
+### Program structure
+
+This section describes the semantic structure of Nix, and how it
+relates to the rest of the specification.
+
+Each Nix program is a single [*expression*](#expressions) denoting a
+[*value*](#values) (commonly a [*function*](#functions)). Each value
+has a [*type*](#types), however this type is not statically known.
+
+Nix code is modularised through the use of the
+[*import*](#builtins-import) built-in function. No separate module
+system exists.
+
+In addition to chapters describing the building blocks mentioned
+above, this specificiation also describes the [*syntax*](#syntax), the
+available [built-in functions](#builtins), [*error handling*](#errors)
+and known [*deficiencies*](#deficiencies) in the language.