This is the gist of how Nixpkgs works (and how it is organized).

QUESTION: What is a fixed-point of a function?

ANSWER: See this video) at least, and the Fixed-point (mathematics)) wikipedia article:

In mathematics, a fixed point (sometimes shortened to fixpoint, also known as an invariant point) of a function is an element of the function's domain that is mapped to itself by the function. That is to say, c is a fixed point of the function f if f(c) = c. This means

f(f(...f(c)...)) = f n(c) = c

an important terminating consideration when recursively computing f. A set of fixed points is sometimes called a fixed set.

For example, if f is defined on the real numbers by f(x)=x^{2}-3x+4,}, then 2 is a fixed point of f, because f(2) = 2.

There is also the wiki article fixed-point combinator that actually plays a role here, but read through the articles in this order.

Then dissect the Stackoverflow thread What is a Y combinator?, and pay attention to the comments! For example:

According to Mike Vanier's description, your definition for Y is actually not a combinator because it's recursive. Under "Eliminating (most) explicit recursion (lazy version)" he has the lazy scheme equivalent of your C# code but explains in point 2: "It is not a combinator, because the Y in the body of the definition is a free variable which is only bound once the definition is complete..." I think the cool thing about Y-combinators is that they produce recursion by evaluating the fixed-point of a function. In this way, they don't need explicit recursion. – GrantJ Jul 18 '11 at 0:02

(wut?)

Other resources in no particular order:

• google search for "fixpoint evaluation fixed point combinator explained"
• Cornell's CS 6110 S17 Lecture 5
• google search for "fixed-point of a function"
• Fixed-point theorem (wikipedia)
• How can I find the fixed points of a function? (math stackexchange)
• The Y Combinator (Slight Return) (see the "Fixpoint of functions" section)
• Clear, intuitive derivation of the fixed-point combinator (Y combinator)? (computer science stackexchange)
• Fixed-Point Combinators (stackoverflow)
• Fixed-point combinator (HandWiki)

QUESTION: How the hell did they come up with the idea of using this with Nix and package management? (..and who? I remember a video saved somewhere, but maybe that was about overlays)

QUESTION: ... and how does it work in this context?

ANSWER: Well, not an answer yet, but this may be something in the right direction:

http://blog.tpleyer.de/posts/2020-01-29-Nix-overlay-evaluation-example.html

QUESTION: Does this refer to pkgs/stdenv/generic/setup.sh? According to 6.5 Phases in the Nixpkgs manual?

ANSWER: I'm pretty sure it does. It sets up the environment (not sure how yet; I see the env vars, but not the basic commands - sed, awk, etc. - that are listed below) and defines a bunch of functions (such as genericBuilder) but it doesn't call these functions!

Didn't they mean what mkDerivation requires?

I have been jumping around in this manual, so not sure about what arguments does derivation require.

The error messages are not helpful when one messes up the input attribute set ofmkDerivation (i.e., either name, or pname and version attributes have to be present); see Nixpkgs issue #113520.

... and from Chapter 11:

fetchpatch works very similarly to fetchurl with the same arguments expected. It expects patch files as a source and and performs normalization on them before computing the checksum. For example it will remove comments or other unstable parts that are sometimes added by version control systems and can change over time.

... and also adding highlight of 19.3. Submitting security fixes

because these are the only places I've seen fetchpatch mentioned.

From the wild in freeswitch/default.nix in Nixpkgs:

stdenv.mkDerivation rec {
  pname = "freeswitch";
  version = "1.10.5";
  src = fetchFromGitHub {
    owner = "signalwire";
    repo = pname;
    rev = "v${version}";
    sha256 = "18dhyb19k28dcm1i8mhqvvgm2phsrmrwyjmfn79glk8pdlalvcha";
  };

  patches = [
    # https://github.com/signalwire/freeswitch/pull/812 fix mod_spandsp, mod_gsmopen build, drop when updating from 1.10.5
    (fetchpatch {
      url = "https://github.com/signalwire/freeswitch/commit/51fba83ed3ed2d9753d8e6b13e13001aca50b493.patch";
      sha256 = "0h2bmifsyyasxjka3pczbmqym1chvz91fmb589njrdbwpkjyvqh3";
    })
  ];
  postPatch = ''
    patchShebangs     libs/libvpx/build/make/rtcd.pl
    substituteInPlace libs/libvpx/build/make/configure.sh \
      --replace AS=\''${AS} AS=yasm

    # Disable advertisement banners
    for f in src/include/cc.h libs/esl/src/include/cc.h; do
      {
        echo 'const char *cc = "";'
        echo 'const char *cc_s = "";'
      } > $f
    done
  '';

Not sure why this isn't called build phases... See also.

nix-build <path> --run-env has been superseded by nix-shell. From Nix manual section C.12. Release 1.6 (2013-09-10):

The command nix-build --run-env has been renamed to nix-shell.

i should remember this to only get last commit, which basically means to only care about the data, and discard commit history.

TODO: Is this the same as lib.attrsets.attrValues in nixpkgs?

TODO Is this an alias to builtins.attrValues?

TODO: document lib.mkIf

Found a description in the NixOS manual:

The special function mkIf causes the evaluation of the conditional to be “pushed down” into the individual definitions, as if you had written:

config = {
  environment.systemPackages = if
config.services.httpd.enable then [ ... ] else [];
  ...
};

TODO: document lib.mkDefault

Not mentioned in any of the manuals (i.e., nix, nixpkgs, NixOS), only the NixOS options page has 1 mention:

Some promising search result (in the order of relevance):

• https://releases.nixos.org/nix-dev/2015-June/017549.html
• https://releases.nixos.org/nix-dev/2015-June/017549.html

From source:

  mkOverride = priority: content:
    { _type = "override";
      inherit priority content;
    };

  mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default

It seems that all the mkOverride calls set up a priority for certain actions.

What is an option?

From the link below to the source, it's just an attribute set with all the function parameters (see below) plus a _type attribute name to allow for type level operations (because Nix is untyped).

  mkOption =
    {
    # Default value used when no definition is given in the configuration.
    default ? null,
    # Textual representation of the default, for the manual.
    defaultText ? null,
    # Example value used in the manual.
    example ? null,
    # String describing the option.
    description ? null,
    # Related packages used in the manual (see `genRelatedPackages` in ../nixos/lib/make-options-doc/default.nix).
    relatedPackages ? null,
    # Option type, providing type-checking and value merging.
    type ? null,
    # Function that converts the option value to something else.
    apply ? null,
    # Whether the option is for NixOS developers only.
    internal ? null,
    # Whether the option shows up in the manual.
    visible ? null,
    # Whether the option can be set only once
    readOnly ? null,
    # Deprecated, used by types.optionSet.
    options ? null
    } @ attrs:
    attrs // { _type = "option"; };

How does it fit in a big scheme of things?

?

If there is recursiveUpdateUntil (i.e., mergeUntil) then mergeWith would also be welcome.

Would be nice to have an alias called merge.

Fix: add semicolon after last Any. (same above in the main type signature)

So the type signature

(String -> Any -> Any) -> AttrSet -> Any
                                     ^^^

should be

(String -> Any -> Any) -> AttrSet -> List
                                     ^^^^

instead, right?

See comment above.

Then why not call it collectVals?... (Following the distinctive naming convention between lib.attrsets.attrVals and lib.attrsets.attrValues.)

Especially because there is lib.attrsets.collect:

catAttrs :: String -> [AttrSet] -> [Any]
collect :: (Any -> Bool) -> AttrSet -> [Any]

(Call it filterVals? There are filterAttrs* functions but those return an attribute set, so no collision.)

This should read "Attribute Set Functions". Nowhere else is attribute sets spelled as "attribute-set".

edit: Alright, "attribute-set" is used in total of 4 times, whereas "attribute set" is used 112 times.

It is kind of an anti-pattern using the angle bracket notation and the linked post also makes a suggestion:

{ pkgs ? import (fetchTarball https://github.com/NixOS/nixpkgs/archive/3590f02e7d5760e52072c1a729ee2250b5560746.tar.gz) {};
}:
# ...

It is kind of an anti-pattern using the angle bracket notation and the linked post also makes a suggestion:

{ pkgs ? import (fetchTarball https://github.com/NixOS/nixpkgs/archive/3590f02e7d5760e52072c1a729ee2250b5560746.tar.gz) {};
}:
# ...

It is kind of an anti-pattern using the angle bracket notation and the linked post also makes a suggestion:

{ pkgs ? import (fetchTarball https://github.com/NixOS/nixpkgs/archive/3590f02e7d5760e52072c1a729ee2250b5560746.tar.gz) {};
}:
# ...

What is the difference between the vim and vim_configurable packages?

I believe the source for the latter is here.

One way (are there others?) to pin Nixpkgs, and ditch channels.

Nix ecosystem introduction ("Talk at my company")

Generates the configuration.nix on iso build