Miscellaneous functions

`lib.trivial.id`

id :: a -> a

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

x: The value to return

`lib.trivial.const`

const :: a -> b -> a

The constant function

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

x: Value to return
y: Value to ignore

lib.trivial.const usage example

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

`lib.trivial.pipe`

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

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

val: Function argument
functions: Function argument

lib.trivial.pipe usage example

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.

`lib.trivial.concat`

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

Concatenate two lists

x: Function argument
y: Function argument

lib.trivial.concat usage example

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

`lib.trivial.or`

boolean “or”

x: Function argument
y: Function argument

`lib.trivial.and`

boolean “and”

x: Function argument
y: Function argument

`lib.trivial.boolToString`

boolToString :: bool -> string

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!).

b: Function argument

`lib.trivial.mergeAttrs`

Merge two attribute sets shallowly, right side trumps left

mergeAttrs :: attrs -> attrs -> attrs

x: Left attribute set
y: Right attribute set (higher precedence for equal keys)

lib.trivial.mergeAttrs usage example

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

`lib.trivial.flip`

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

Flip the order of the arguments of a binary function.

f: Function argument
a: Function argument
b: Function argument

lib.trivial.flip usage example

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

`lib.trivial.mapNullable`

Apply function if the supplied argument is non-null.

f: Function to call
a: Argument to check for null before passing it to f

lib.trivial.mapNullable usage example

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

`lib.trivial.version`

Returns the current full nixpkgs version number.

`lib.trivial.release`

Returns the current nixpkgs release number as string.

`lib.trivial.oldestSupportedRelease`

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.

`lib.trivial.isInOldestRelease`

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

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.

`lib.trivial.codeName`

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.

`lib.trivial.versionSuffix`

Returns the current nixpkgs version suffix as string.

`lib.trivial.revisionWithDefault`

revisionWithDefault :: string -> string

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

default: Default value to return if revision can not be determined

`lib.trivial.inNixShell`

inNixShell :: bool

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

`lib.trivial.inPureEvalMode`

inPureEvalMode :: bool

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.

`lib.trivial.min`

Return minimum of two numbers.

x: Function argument
y: Function argument

`lib.trivial.max`

Return maximum of two numbers.

x: Function argument
y: Function argument

`lib.trivial.mod`

Integer modulus

base: Function argument
int: Function argument

lib.trivial.mod usage example

mod 11 10
=> 1
mod 1 10
=> 1

`lib.trivial.compare`

C-style comparisons

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

a: Function argument
b: Function argument

`lib.trivial.splitByAndCompare`

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

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.

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

lib.trivial.splitByAndCompare usage example

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

`lib.trivial.importJSON`

Reads a JSON file.

Type :: path -> any

path: Function argument

`lib.trivial.importTOML`

Reads a TOML file.

Type :: path -> any

path: Function argument

`lib.trivial.warn`

string -> a -> a

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

`lib.trivial.warnIf`

bool -> string -> a -> a

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

cond: Function argument
msg: Function argument

`lib.trivial.warnIfNot`

bool -> string -> a -> a

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

cond: Function argument
msg: Function argument

`lib.trivial.throwIfNot`

bool -> string -> a -> a

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.

cond: Function argument
msg: Function argument

lib.trivial.throwIfNot usage example


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

`lib.trivial.throwIf`

bool -> string -> a -> a

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

cond: Function argument
msg: Function argument

`lib.trivial.checkListOfEnum`

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

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

msg: Function argument
valid: Function argument
given: Function argument

lib.trivial.checkListOfEnum usage example

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

`lib.trivial.setFunctionArgs`

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.

f: Function argument
args: Function argument

`lib.trivial.functionArgs`

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.

f: Function argument

`lib.trivial.isFunction`

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

f: Function argument

`lib.trivial.toFunction`

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

v: Any value

lib.trivial.toFunction usage example


nix-repl> lib.toFunction 1 2
1

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

`lib.trivial.toHexString`

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

toHexString 0 => "0"

toHexString 16 => "10"

toHexString 250 => "FA"

i: Function argument

`lib.trivial.toBaseDigits`

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 ]

base: Function argument
i: Function argument