{ lib }:

let
  inherit (lib.trivial)
    isFunction
    isInt
    functionArgs
    pathExists
    release
    setFunctionArgs
    toBaseDigits
    version
    versionSuffix
    warn;
in {

  ## Simple (higher order) functions

  /**
    The identity function
    For when you need a function that does “nothing”.


    # Inputs

    `x`

    : The value to return

    # Type

    ```
    id :: a -> a
    ```
  */
  id = x: x;

  /**
    The constant function

    Ignores the second argument. If called with only one argument,
    constructs a function that always returns a static value.


    # Inputs

    `x`

    : Value to return

    `y`

    : Value to ignore

    # Type

    ```
    const :: a -> b -> a
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.const` usage example

    ```nix
    let f = const 5; in f 10
    => 5
    ```

    :::
  */
  const =
    x:
    y: x;

  /**
    Pipes a value through a list of functions, left to right.

    # Inputs

    `value`

    : Value to start piping.

    `fns`

    : List of functions to apply sequentially.

    # Type

    ```
    pipe :: a -> [<functions>] -> <return type of last function>
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.pipe` usage example

    ```nix
    pipe 2 [
        (x: x + 2)  # 2 + 2 = 4
        (x: x * 2)  # 4 * 2 = 8
      ]
    => 8

    # ideal to do text transformations
    pipe [ "a/b" "a/c" ] [

      # create the cp command
      (map (file: ''cp "${src}/${file}" $out\n''))

      # concatenate all commands into one string
      lib.concatStrings

      # make that string into a nix derivation
      (pkgs.runCommand "copy-to-out" {})

    ]
    => <drv which copies all files to $out>

    The output type of each function has to be the input type
    of the next function, and the last function returns the
    final value.
    ```

    :::
  */
  pipe = builtins.foldl' (x: f: f x);

  # note please don’t add a function like `compose = flip pipe`.
  # This would confuse users, because the order of the functions
  # in the list is not clear. With pipe, it’s obvious that it
  # goes first-to-last. With `compose`, not so much.

  ## Named versions corresponding to some builtin operators.

  /**
    Concatenate two lists


    # Inputs

    `x`

    : 1\. Function argument

    `y`

    : 2\. Function argument

    # Type

    ```
    concat :: [a] -> [a] -> [a]
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.concat` usage example

    ```nix
    concat [ 1 2 ] [ 3 4 ]
    => [ 1 2 3 4 ]
    ```

    :::
  */
  concat = x: y: x ++ y;

  /**
    boolean “or”


    # Inputs

    `x`

    : 1\. Function argument

    `y`

    : 2\. Function argument
  */
  or = x: y: x || y;

  /**
    boolean “and”


    # Inputs

    `x`

    : 1\. Function argument

    `y`

    : 2\. Function argument
  */
  and = x: y: x && y;

  /**
    bitwise “not”
  */
  bitNot = builtins.sub (-1);

  /**
    Convert a boolean to a string.

    This function uses the strings "true" and "false" to represent
    boolean values. Calling `toString` on a bool instead returns "1"
    and "" (sic!).


    # Inputs

    `b`

    : 1\. Function argument

    # Type

    ```
    boolToString :: bool -> string
    ```
  */
  boolToString = b: if b then "true" else "false";

  /**
    Merge two attribute sets shallowly, right side trumps left

    mergeAttrs :: attrs -> attrs -> attrs


    # Inputs

    `x`

    : Left attribute set

    `y`

    : Right attribute set (higher precedence for equal keys)


    # Examples
    :::{.example}
    ## `lib.trivial.mergeAttrs` usage example

    ```nix
    mergeAttrs { a = 1; b = 2; } { b = 3; c = 4; }
    => { a = 1; b = 3; c = 4; }
    ```

    :::
  */
  mergeAttrs =
    x:
    y: x // y;

  /**
    Flip the order of the arguments of a binary function.


    # Inputs

    `f`

    : 1\. Function argument

    `a`

    : 2\. Function argument

    `b`

    : 3\. Function argument

    # Type

    ```
    flip :: (a -> b -> c) -> (b -> a -> c)
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.flip` usage example

    ```nix
    flip concat [1] [2]
    => [ 2 1 ]
    ```

    :::
  */
  flip = f: a: b: f b a;

  /**
    Apply function if the supplied argument is non-null.


    # Inputs

    `f`

    : Function to call

    `a`

    : Argument to check for null before passing it to `f`


    # Examples
    :::{.example}
    ## `lib.trivial.mapNullable` usage example

    ```nix
    mapNullable (x: x+1) null
    => null
    mapNullable (x: x+1) 22
    => 23
    ```

    :::
  */
  mapNullable =
    f:
    a: if a == null then a else f a;

  # Pull in some builtins not included elsewhere.
  inherit (builtins)
    pathExists readFile isBool
    isInt isFloat add sub lessThan
    seq deepSeq genericClosure
    bitAnd bitOr bitXor;

  ## nixpkgs version strings

  /**
    Returns the current full nixpkgs version number.
  */
  version = release + versionSuffix;

  /**
    Returns the current nixpkgs release number as string.
  */
  release = lib.strings.fileContents ./.version;

  /**
    The latest release that is supported, at the time of release branch-off,
    if applicable.

    Ideally, out-of-tree modules should be able to evaluate cleanly with all
    supported Nixpkgs versions (master, release and old release until EOL).
    So if possible, deprecation warnings should take effect only when all
    out-of-tree expressions/libs/modules can upgrade to the new way without
    losing support for supported Nixpkgs versions.

    This release number allows deprecation warnings to be implemented such that
    they take effect as soon as the oldest release reaches end of life.
  */
  oldestSupportedRelease =
    # Update on master only. Do not backport.
    2311;

  /**
    Whether a feature is supported in all supported releases (at the time of
    release branch-off, if applicable). See `oldestSupportedRelease`.


    # Inputs

    `release`

    : Release number of feature introduction as an integer, e.g. 2111 for 21.11.
    Set it to the upcoming release, matching the nixpkgs/.version file.
  */
  isInOldestRelease =
    release:
      release <= lib.trivial.oldestSupportedRelease;

  /**
    Returns the current nixpkgs release code name.

    On each release the first letter is bumped and a new animal is chosen
    starting with that new letter.
  */
  codeName = "Uakari";

  /**
    Returns the current nixpkgs version suffix as string.
  */
  versionSuffix =
    let suffixFile = ../.version-suffix;
    in if pathExists suffixFile
    then lib.strings.fileContents suffixFile
    else "pre-git";

  /**
    Attempts to return the the current revision of nixpkgs and
    returns the supplied default value otherwise.


    # Inputs

    `default`

    : Default value to return if revision can not be determined

    # Type

    ```
    revisionWithDefault :: string -> string
    ```
  */
  revisionWithDefault =
    default:
    let
      revisionFile = "${toString ./..}/.git-revision";
      gitRepo      = "${toString ./..}/.git";
    in if lib.pathIsGitRepo gitRepo
       then lib.commitIdFromGitRepo gitRepo
       else if lib.pathExists revisionFile then lib.fileContents revisionFile
       else default;

  nixpkgsVersion = warn "lib.nixpkgsVersion is a deprecated alias of lib.version." version;

  /**
    Determine whether the function is being called from inside a Nix
    shell.

    # Type

    ```
    inNixShell :: bool
    ```
  */
  inNixShell = builtins.getEnv "IN_NIX_SHELL" != "";

  /**
    Determine whether the function is being called from inside pure-eval mode
    by seeing whether `builtins` contains `currentSystem`. If not, we must be in
    pure-eval mode.

    # Type

    ```
    inPureEvalMode :: bool
    ```
  */
  inPureEvalMode = ! builtins ? currentSystem;

  ## Integer operations

  /**
    Return minimum of two numbers.


    # Inputs

    `x`

    : 1\. Function argument

    `y`

    : 2\. Function argument
  */
  min = x: y: if x < y then x else y;

  /**
    Return maximum of two numbers.


    # Inputs

    `x`

    : 1\. Function argument

    `y`

    : 2\. Function argument
  */
  max = x: y: if x > y then x else y;

  /**
    Integer modulus


    # Inputs

    `base`

    : 1\. Function argument

    `int`

    : 2\. Function argument


    # Examples
    :::{.example}
    ## `lib.trivial.mod` usage example

    ```nix
    mod 11 10
    => 1
    mod 1 10
    => 1
    ```

    :::
  */
  mod = base: int: base - (int * (builtins.div base int));


  ## Comparisons

  /**
    C-style comparisons

    a < b,  compare a b => -1
    a == b, compare a b => 0
    a > b,  compare a b => 1


    # Inputs

    `a`

    : 1\. Function argument

    `b`

    : 2\. Function argument
  */
  compare = a: b:
    if a < b
    then -1
    else if a > b
         then 1
         else 0;

  /**
    Split type into two subtypes by predicate `p`, take all elements
    of the first subtype to be less than all the elements of the
    second subtype, compare elements of a single subtype with `yes`
    and `no` respectively.


    # Inputs

    `p`

    : Predicate

    `yes`

    : Comparison function if predicate holds for both values

    `no`

    : Comparison function if predicate holds for neither value

    `a`

    : First value to compare

    `b`

    : Second value to compare

    # Type

    ```
    (a -> bool) -> (a -> a -> int) -> (a -> a -> int) -> (a -> a -> int)
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.splitByAndCompare` usage example

    ```nix
    let cmp = splitByAndCompare (hasPrefix "foo") compare compare; in

    cmp "a" "z" => -1
    cmp "fooa" "fooz" => -1

    cmp "f" "a" => 1
    cmp "fooa" "a" => -1
    # while
    compare "fooa" "a" => 1
    ```

    :::
  */
  splitByAndCompare =
    p: yes: no: a: b:
    if p a
    then if p b then yes a b else -1
    else if p b then 1 else no a b;


  /**
    Reads a JSON file.


    # Inputs

    `path`

    : 1\. Function argument

    # Type

    ```
    importJSON :: path -> any
    ```
  */
  importJSON = path:
    builtins.fromJSON (builtins.readFile path);

  /**
    Reads a TOML file.


    # Inputs

    `path`

    : 1\. Function argument

    # Type

    ```
    importTOML :: path -> any
    ```
  */
  importTOML = path:
    builtins.fromTOML (builtins.readFile path);

  ## Warnings

  # See https://github.com/NixOS/nix/issues/749. Eventually we'd like these
  # to expand to Nix builtins that carry metadata so that Nix can filter out
  # the INFO messages without parsing the message string.
  #
  # Usage:
  # {
  #   foo = lib.warn "foo is deprecated" oldFoo;
  #   bar = lib.warnIf (bar == "") "Empty bar is deprecated" bar;
  # }
  #
  # TODO: figure out a clever way to integrate location information from
  # something like __unsafeGetAttrPos.

  /**
    Print a warning before returning the second argument. This function behaves
    like `builtins.trace`, but requires a string message and formats it as a
    warning, including the `warning: ` prefix.

    To get a call stack trace and abort evaluation, set the environment variable
    `NIX_ABORT_ON_WARN=true` and set the Nix options `--option pure-eval false --show-trace`

    # Inputs

    `msg`

    : Warning message to print.

    `val`

    : Value to return as-is.

    # Type

    ```
    string -> a -> a
    ```
  */
  warn =
    if lib.elem (builtins.getEnv "NIX_ABORT_ON_WARN") ["1" "true" "yes"]
    then msg: builtins.trace "[1;31mwarning: ${msg}[0m" (abort "NIX_ABORT_ON_WARN=true; warnings are treated as unrecoverable errors.")
    else msg: builtins.trace "[1;31mwarning: ${msg}[0m";

  /**
    Like warn, but only warn when the first argument is `true`.


    # Inputs

    `cond`

    : 1\. Function argument

    `msg`

    : 2\. Function argument

    `val`

    : Value to return as-is.

    # Type

    ```
    bool -> string -> a -> a
    ```
  */
  warnIf = cond: msg: if cond then warn msg else x: x;

  /**
    Like warnIf, but negated (warn if the first argument is `false`).


    # Inputs

    `cond`

    : 1\. Function argument

    `msg`

    : 2\. Function argument

    `val`

    : Value to return as-is.

    # Type

    ```
    bool -> string -> a -> a
    ```
  */
  warnIfNot = cond: msg: if cond then x: x else warn msg;

  /**
    Like the `assert b; e` expression, but with a custom error message and
    without the semicolon.

    If true, return the identity function, `r: r`.

    If false, throw the error message.

    Calls can be juxtaposed using function application, as `(r: r) a = a`, so
    `(r: r) (r: r) a = a`, and so forth.


    # Inputs

    `cond`

    : 1\. Function argument

    `msg`

    : 2\. Function argument

    # Type

    ```
    bool -> string -> a -> a
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.throwIfNot` usage example

    ```nix
    throwIfNot (lib.isList overlays) "The overlays argument to nixpkgs must be a list."
    lib.foldr (x: throwIfNot (lib.isFunction x) "All overlays passed to nixpkgs must be functions.") (r: r) overlays
    pkgs
    ```

    :::
  */
  throwIfNot = cond: msg: if cond then x: x else throw msg;

  /**
    Like throwIfNot, but negated (throw if the first argument is `true`).


    # Inputs

    `cond`

    : 1\. Function argument

    `msg`

    : 2\. Function argument

    # Type

    ```
    bool -> string -> a -> a
    ```
  */
  throwIf = cond: msg: if cond then throw msg else x: x;

  /**
    Check if the elements in a list are valid values from a enum, returning the identity function, or throwing an error message otherwise.


    # Inputs

    `msg`

    : 1\. Function argument

    `valid`

    : 2\. Function argument

    `given`

    : 3\. Function argument

    # Type

    ```
    String -> List ComparableVal -> List ComparableVal -> a -> a
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.checkListOfEnum` usage example

    ```nix
    let colorVariants = ["bright" "dark" "black"]
    in checkListOfEnum "color variants" [ "standard" "light" "dark" ] colorVariants;
    =>
    error: color variants: bright, black unexpected; valid ones: standard, light, dark
    ```

    :::
  */
  checkListOfEnum = msg: valid: given:
    let
      unexpected = lib.subtractLists valid given;
    in
      lib.throwIfNot (unexpected == [])
        "${msg}: ${builtins.concatStringsSep ", " (builtins.map builtins.toString unexpected)} unexpected; valid ones: ${builtins.concatStringsSep ", " (builtins.map builtins.toString valid)}";

  info = msg: builtins.trace "INFO: ${msg}";

  showWarnings = warnings: res: lib.foldr (w: x: warn w x) res warnings;

  ## Function annotations

  /**
    Add metadata about expected function arguments to a function.
    The metadata should match the format given by
    builtins.functionArgs, i.e. a set from expected argument to a bool
    representing whether that argument has a default or not.
    setFunctionArgs : (a → b) → Map String Bool → (a → b)

    This function is necessary because you can't dynamically create a
    function of the { a, b ? foo, ... }: format, but some facilities
    like callPackage expect to be able to query expected arguments.


    # Inputs

    `f`

    : 1\. Function argument

    `args`

    : 2\. Function argument
  */
  setFunctionArgs = f: args:
    { # TODO: Should we add call-time "type" checking like built in?
      __functor = self: f;
      __functionArgs = args;
    };

  /**
    Extract the expected function arguments from a function.
    This works both with nix-native { a, b ? foo, ... }: style
    functions and functions with args set with 'setFunctionArgs'. It
    has the same return type and semantics as builtins.functionArgs.
    setFunctionArgs : (a → b) → Map String Bool.


    # Inputs

    `f`

    : 1\. Function argument
  */
  functionArgs = f:
    if f ? __functor
    then f.__functionArgs or (functionArgs (f.__functor f))
    else builtins.functionArgs f;

  /**
    Check whether something is a function or something
    annotated with function args.


    # Inputs

    `f`

    : 1\. Function argument
  */
  isFunction = f: builtins.isFunction f ||
    (f ? __functor && isFunction (f.__functor f));

  /**
    `mirrorFunctionArgs f g` creates a new function `g'` with the same behavior as `g` (`g' x == g x`)
    but its function arguments mirroring `f` (`lib.functionArgs g' == lib.functionArgs f`).


    # Inputs

    `f`

    : Function to provide the argument metadata

    `g`

    : Function to set the argument metadata to

    # Type

    ```
    mirrorFunctionArgs :: (a -> b) -> (a -> c) -> (a -> c)
    ```

    # Examples
    :::{.example}
    ## `lib.trivial.mirrorFunctionArgs` usage example

    ```nix
    addab = {a, b}: a + b
    addab { a = 2; b = 4; }
    => 6
    lib.functionArgs addab
    => { a = false; b = false; }
    addab1 = attrs: addab attrs + 1
    addab1 { a = 2; b = 4; }
    => 7
    lib.functionArgs addab1
    => { }
    addab1' = lib.mirrorFunctionArgs addab addab1
    addab1' { a = 2; b = 4; }
    => 7
    lib.functionArgs addab1'
    => { a = false; b = false; }
    ```

    :::
  */
  mirrorFunctionArgs =
    f:
    let
      fArgs = functionArgs f;
    in
    g:
    setFunctionArgs g fArgs;

  /**
    Turns any non-callable values into constant functions.
    Returns callable values as is.


    # Inputs

    `v`

    : Any value


    # Examples
    :::{.example}
    ## `lib.trivial.toFunction` usage example

    ```nix
    nix-repl> lib.toFunction 1 2
    1

    nix-repl> lib.toFunction (x: x + 1) 2
    3
    ```

    :::
  */
  toFunction =
    v:
    if isFunction v
    then v
    else k: v;

  /**
    Convert the given positive integer to a string of its hexadecimal
    representation. For example:

    toHexString 0 => "0"

    toHexString 16 => "10"

    toHexString 250 => "FA"
  */
  toHexString = let
    hexDigits = {
      "10" = "A";
      "11" = "B";
      "12" = "C";
      "13" = "D";
      "14" = "E";
      "15" = "F";
    };
    toHexDigit = d:
      if d < 10
      then toString d
      else hexDigits.${toString d};
  in i: lib.concatMapStrings toHexDigit (toBaseDigits 16 i);

  /**
    `toBaseDigits base i` converts the positive integer i to a list of its
    digits in the given base. For example:

    toBaseDigits 10 123 => [ 1 2 3 ]

    toBaseDigits 2 6 => [ 1 1 0 ]

    toBaseDigits 16 250 => [ 15 10 ]


    # Inputs

    `base`

    : 1\. Function argument

    `i`

    : 2\. Function argument
  */
  toBaseDigits = base: i:
    let
      go = i:
        if i < base
        then [i]
        else
          let
            r = i - ((i / base) * base);
            q = (i - r) / base;
          in
            [r] ++ go q;
    in
      assert (isInt base);
      assert (isInt i);
      assert (base >= 2);
      assert (i >= 0);
      lib.reverseList (go i);
}
