nixpkgs/lib/trivial.nix at 718f670e845328e9c2cecdd832835712b2cbd065 · NixOS/nixpkgs

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{ lib }:

let

inherit (lib.trivial)

isFunction

isInt

functionArgs

pathExists

release

toBaseDigits

version

versionSuffix

warn

;

inherit (lib)

foldr

fromJSON

isString

readFile

;

{

# Pull in some builtins not included elsewhere.

inherit (builtins)

pathExists

readFile

isBool

isInt

isFloat

add

sub

mul

div

lessThan

seq

deepSeq

genericClosure

bitAnd

bitOr

bitXor

ceil

floor

;

## 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 -> [(a -> b) (b -> c) ... (x -> y) (y -> z)] -> z

```

# 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

# Type

```

or :: Bool -> Bool -> Bool

```

"or" = x: y: x || y;

/**

boolean “and”

# Inputs

`x`

: 1\. Function argument

`y`

: 2\. Function argument

# Type

```

and :: Bool -> Bool -> Bool

```

and = x: y: x && y;

/**

boolean “exclusive or”

# Inputs

`x`

: 1\. Function argument

`y`

: 2\. Function argument

# Type

```

xor :: bool -> bool -> bool

```

# We explicitly invert the arguments purely as a type assertion.

# This is invariant under XOR, so it does not affect the result.

xor = x: y: (!x) != (!y);

/**

bitwise “not”

# Type

```

bitNot :: Number -> Number

```

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";

/**

Converts a boolean to a string.

This function uses the strings "yes" and "no" to represent

boolean values.

# Inputs

`b`

: The boolean to convert

# Type

```

boolToYesNo :: Bool -> String

```

boolToYesNo = b: if b then "yes" else "no";

/**

Merge two attribute sets shallowly, right side trumps left

# Inputs

`x`

: Left attribute set

`y`

: Right attribute set (higher precedence for equal keys)

# Type

```

mergeAttrs :: AttrSet -> AttrSet -> AttrSet

```

# 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;

/**

Returns `maybeValue` if not null, otherwise return `default`.

# Inputs

`default`

: 1\. Function argument

`maybeValue`

: 2\. Function argument

# Type

```

defaultTo :: a -> (b | Null) -> (b | a)

```

# Examples

:::{.example}

## `lib.trivial.defaultTo` usage example

```nix

defaultTo "default" null

=> "default"

defaultTo "default" "foo"

=> "foo"

defaultTo "default" false

=> false

```

:::

defaultTo = default: maybeValue: if maybeValue != null then maybeValue else default;

/**

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`

# Type

```

mapNullable :: (a -> b) -> (a | Null) -> (b | Null)

```

# 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;

## 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.

2511;

/**

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 =

lib.warnIf (lib.oldestSupportedReleaseIsAtLeast 2411)

"lib.isInOldestRelease is deprecated. Use lib.oldestSupportedReleaseIsAtLeast instead."

lib.oldestSupportedReleaseIsAtLeast;

/**

Alias for `isInOldestRelease` introduced in 24.11.

Use `isInOldestRelease` in expressions outside of Nixpkgs for greater compatibility.

oldestSupportedReleaseIsAtLeast = 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 = "Yarara";

/**

Returns the current nixpkgs version suffix as string.

versionSuffix =

let

suffixFile = ../.version-suffix;

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";

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

/**

Returns minimum of two numbers.

# Inputs

`x`

: 1\. Function argument

`y`

: 2\. Function argument

# Type

```

min :: Number -> Number -> Number

```

min = x: y: if x < y then x else y;

/**

Returns maximum of two numbers.

# Inputs

`x`

: 1\. Function argument

`y`

: 2\. Function argument

# Type

```

max :: Number -> Number -> Number

```

max = x: y: if x > y then x else y;

/**

Integer modulus

# Inputs

`base`

: 1\. Function argument

`int`

: 2\. Function argument

# Type

```

mod :: Int -> Int -> Int

```

# 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

# Type

```

compare :: a -> a -> Int

```

compare =

a: b:

if a < b then

-1

else if a > b then

else

/**

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

```

splitByAndCompare :: (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

else

no a b;

/**

Reads a JSON file.

# Examples

:::{.example}

## `lib.trivial.importJSON` usage example

example.json

```json

{

"title": "Example JSON",

"hello": {

"world": "foo",

"bar": {

"foobar": true

}

```

```nix

importJSON ./example.json

=> {

title = "Example JSON";

hello = {

world = "foo";

bar = {

foobar = true;

};

}

```

:::

# Inputs

`path`

: 1\. Function argument

# Type

```

importJSON :: Path -> Any

```

importJSON = path: fromJSON (readFile path);

/**

Reads a TOML file.

# Examples

:::{.example}

## `lib.trivial.importTOML` usage example

example.toml

```toml

title = "TOML Example"

[hello]

world = "foo"

[hello.bar]

foobar = true

```

```nix

importTOML ./example.toml

=> {

title = "TOML Example";

hello = {

world = "foo";

bar = {

foobar = true;

};

}

```

:::

# Inputs

`path`

: 1\. Function argument

# Type

```

importTOML :: Path -> Any

```

importTOML = path: fromTOML (readFile path);

/**

`warn` *`message`* *`value`*

Print a warning before returning the second argument.

See [`builtins.warn`](https://nix.dev/manual/nix/latest/language/builtins.html#builtins-warn) (Nix >= 2.23).

On older versions, the Nix 2.23 behavior is emulated with [`builtins.trace`](https://nix.dev/manual/nix/latest/language/builtins.html#builtins-warn), including the [`NIX_ABORT_ON_WARN`](https://nix.dev/manual/nix/latest/command-ref/conf-file#conf-abort-on-warn) behavior, but not the `nix.conf` setting or command line option.

# Inputs

*`message`* (String)

: Warning message to print before evaluating *`value`*.

*`value`* (any value)

: Value to return as-is.

# Type

```

warn :: String -> a -> a

```

warn =

# Since Nix 2.23, https://github.com/NixOS/nix/pull/10592

builtins.warn or (

let

mustAbort = lib.elem (builtins.getEnv "NIX_ABORT_ON_WARN") [

"1"

"true"

"yes"

];

# Do not eta reduce v, so that we have the same strictness as `builtins.warn`.

msg: v:

# `builtins.warn` requires a string message, so we enforce that in our implementation, so that callers aren't accidentally incompatible with newer Nix versions.

assert isString msg;

if mustAbort then

builtins.trace "[1;31mevaluation warning:[0m ${msg}" (

abort "NIX_ABORT_ON_WARN=true; warnings are treated as unrecoverable errors."

)

else

builtins.trace "[1;35mevaluation warning:[0m ${msg}" v

);

/**

`warnIf` *`condition`* *`message`* *`value`*

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

# Inputs

*`condition`* (Boolean)

: `true` to trigger the warning before continuing with *`value`*.

*`message`* (String)

: Warning message to print before evaluating

*`value`* (any value)

: Value to return as-is.

# Type

```

warnIf :: Bool -> String -> a -> a

```

warnIf = cond: msg: if cond then warn msg else x: x;

/**

`warnIfNot` *`condition`* *`message`* *`value`*

Like `warnIf`, but negated: warn if the first argument is `false`.

# Inputs

*`condition`*

: `false` to trigger the warning before continuing with `val`.

*`message`*

: Warning message to print before evaluating *`value`*.

*`value`*

: Value to return as-is.

# Type

```

warnIfNot :: 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

```

throwIfNot :: Bool -> String -> a -> (a | Never)

```

# 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

```

throwIf :: Bool -> String -> a -> (a | Never)

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