Throw if pred is false, else return pred. Intended to be used to augment asserts with helpful error messages.

assertMsg :: Bool -> String -> Bool

assertMsg false "nope"
stderr> error: nope
assert assertMsg ("foo" == "bar") "foo is not bar, silly"; ""
stderr> error: foo is not bar, silly

Specialized assertMsg for checking if val is one of the elements of the list xs. Useful for checking enums.

assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool

let sslLibrary = "libressl";
in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
stderr> error: sslLibrary must be one of [
stderr>   "openssl"
stderr>   "bearssl"
stderr> ], but is: "libressl"

Specialized assertMsg for checking if every one of vals is one of the elements of the list xs. Useful for checking lists of supported attributes.

assertEachOneOf :: String -> List ComparableVal -> List ComparableVal -> Bool

let sslLibraries = [ "libressl" "bearssl" ];
in assertEachOneOf "sslLibraries" sslLibraries [ "openssl" "bearssl" ]
stderr> error: each element in sslLibraries must be one of [
stderr>   "openssl"
stderr>   "bearssl"
stderr> ], but is: [
stderr>   "libressl"
stderr>   "bearssl"
stderr> ]

Return an attribute from nested attribute sets.

Nix has an attribute selection operator . or which is sufficient for such queries, as long as the number of attributes is static. For example:

(x.a.b or 6) == attrByPath ["a" "b"] 6 x
### and
(x.${f p}."example.com" or 6) == attrByPath [ (f p) "example.com" ] 6 x

attrByPath :: [String] -> Any -> AttrSet -> Any

x = { a = { b = 3; }; }
### ["a" "b"] is equivalent to x.a.b
### 6 is a default value to return if the path does not exist in attrset
attrByPath ["a" "b"] 6 x
=> 3
attrByPath ["z" "z"] 6 x
=> 6

Return if an attribute from nested attribute set exists.

Nix has a has attribute operator ?, which is sufficient for such queries, as long as the number of attributes is static. For example:

(x?a.b) == hasAttrByPath ["a" "b"] x
### and
(x?${f p}."example.com") == hasAttrByPath [ (f p) "example.com" ] x

Laws:

hasAttrByPath :: [String] -> AttrSet -> Bool

x = { a = { b = 3; }; }
hasAttrByPath ["a" "b"] x
=> true
hasAttrByPath ["z" "z"] x
=> false
hasAttrByPath [] (throw "no need")
=> true

Return the longest prefix of an attribute path that refers to an existing attribute in a nesting of attribute sets.

Can be used after mapAttrsRecursiveCond to apply a condition, although this will evaluate the predicate function on sibling attributes as well.

Note that the empty attribute path is valid for all values, so this function only throws an exception if any of its inputs does.

Laws:

attrsets.longestValidPathPrefix :: [String] -> Value -> [String]

x = { a = { b = 3; }; }
attrsets.longestValidPathPrefix ["a" "b" "c"] x
=> ["a" "b"]
attrsets.longestValidPathPrefix ["a"] x
=> ["a"]
attrsets.longestValidPathPrefix ["z" "z"] x
=> []
attrsets.longestValidPathPrefix ["z" "z"] (throw "no need")
=> []

Create a new attribute set with value set at the nested attribute location specified in attrPath.

setAttrByPath :: [String] -> Any -> AttrSet

setAttrByPath ["a" "b"] 3
=> { a = { b = 3; }; }

Like attrByPath, but without a default value. If it doesn’t find the path it will throw an error.

Nix has an attribute selection operator which is sufficient for such queries, as long as the number of attributes is static. For example:

x.a.b == getAttrByPath ["a" "b"] x
### and
x.${f p}."example.com" == getAttrByPath [ (f p) "example.com" ] x

getAttrFromPath :: [String] -> AttrSet -> Any

x = { a = { b = 3; }; }
getAttrFromPath ["a" "b"] x
=> 3
getAttrFromPath ["z" "z"] x
=> error: cannot find attribute `z.z'

Map each attribute in the given set and merge them into a new attribute set.

concatMapAttrs :: (String -> a -> AttrSet) -> AttrSet -> AttrSet

concatMapAttrs
  (name: value: {
    ${name} = value;
    ${name + value} = value;
  })
  { x = "a"; y = "b"; }
=> { x = "a"; xa = "a"; y = "b"; yb = "b"; }

Update or set specific paths of an attribute set.

Takes a list of updates to apply and an attribute set to apply them to, and returns the attribute set with the updates applied. Updates are represented as { path = ...; update = ...; } values, where path is a list of strings representing the attribute path that should be updated, and update is a function that takes the old value at that attribute path as an argument and returns the new value it should be.

Properties:

updateManyAttrsByPath :: [{ path :: [String]; update :: (Any -> Any); }] -> AttrSet -> AttrSet

updateManyAttrsByPath [
  {
    path = [ "a" "b" ];
    update = old: { d = old.c; };
  }
  {
    path = [ "a" "b" "c" ];
    update = old: old + 1;
  }
  {
    path = [ "x" "y" ];
    update = old: "xy";
  }
] { a.b.c = 0; }
=> { a = { b = { d = 1; }; }; x = { y = "xy"; }; }

Return the specified attributes from a set.

attrVals :: [String] -> AttrSet -> [Any]

attrVals ["a" "b" "c"] as
=> [as.a as.b as.c]

Return the values of all attributes in the given set, sorted by attribute name.

attrValues :: AttrSet -> [Any]

attrValues {c = 3; a = 1; b = 2;}
=> [1 2 3]

Given a set of attribute names, return the set of the corresponding attributes from the given set.

getAttrs :: [String] -> AttrSet -> AttrSet

getAttrs [ "a" "b" ] { a = 1; b = 2; c = 3; }
=> { a = 1; b = 2; }

Collect each attribute named attr from a list of attribute sets. Sets that don’t contain the named attribute are ignored.

catAttrs :: String -> [AttrSet] -> [Any]

catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
=> [1 2]

Filter an attribute set by removing all attributes for which the given predicate return false.

filterAttrs :: (String -> Any -> Bool) -> AttrSet -> AttrSet

filterAttrs (n: v: n == "foo") { foo = 1; bar = 2; }
=> { foo = 1; }

Filter an attribute set recursively by removing all attributes for which the given predicate return false.

filterAttrsRecursive :: (String -> Any -> Bool) -> AttrSet -> AttrSet

filterAttrsRecursive (n: v: v != null) { foo = { bar = null; }; }
=> { foo = {}; }

Like lib.lists.foldl' but for attribute sets. Iterates over every name-value pair in the given attribute set. The result of the callback function is often called acc for accumulator. It is passed between callbacks from left to right and the final acc is the return value of foldlAttrs.

Attention:

There is a completely different function lib.foldAttrs which has nothing to do with this function, despite the similar name.

foldlAttrs :: ( a -> String -> b -> a ) -> a -> { ... :: b } -> a

foldlAttrs
  (acc: name: value: {
    sum = acc.sum + value;
    names = acc.names ++ [name];
  })
  { sum = 0; names = []; }
  {
    foo = 1;
    bar = 10;
  }
->
  {
    sum = 11;
    names = ["bar" "foo"];
  }

foldlAttrs
  (throw "function not needed")
  123
  {};
->
  123

foldlAttrs
  (acc: _: _: acc)
  3
  { z = throw "value not needed"; a = throw "value not needed"; };
->
  3

The accumulator doesn't have to be an attrset.
It can be as simple as a number or string.

foldlAttrs
  (acc: _: v: acc * 10 + v)
  1
  { z = 1; a = 2; };
->
  121

Apply fold functions to values grouped by key.

foldAttrs :: (Any -> Any -> Any) -> Any -> [AttrSets] -> Any

foldAttrs (item: acc: [item] ++ acc) [] [{ a = 2; } { a = 3; }]
=> { a = [ 2 3 ]; }

Recursively collect sets that verify a given predicate named pred from the set attrs. The recursion is stopped when the predicate is verified.

collect :: (AttrSet -> Bool) -> AttrSet -> [x]

collect isList { a = { b = ["b"]; }; c = [1]; }
=> [["b"] [1]]

collect (x: x ? outPath)
   { a = { outPath = "a/"; }; b = { outPath = "b/"; }; }
=> [{ outPath = "a/"; } { outPath = "b/"; }]

Return the cartesian product of attribute set value combinations.

cartesianProduct :: AttrSet -> [AttrSet]

cartesianProduct { a = [ 1 2 ]; b = [ 10 20 ]; }
=> [
     { a = 1; b = 10; }
     { a = 1; b = 20; }
     { a = 2; b = 10; }
     { a = 2; b = 20; }
   ]

Return the result of function f applied to the cartesian product of attribute set value combinations. Equivalent to using cartesianProduct followed by map.

mapCartesianProduct :: (AttrSet -> a) -> AttrSet -> [a]

mapCartesianProduct ({a, b}: "${a}-${b}") { a = [ "1" "2" ]; b = [ "3" "4" ]; }
=> [ "1-3" "1-4" "2-3" "2-4" ]

Utility function that creates a {name, value} pair as expected by builtins.listToAttrs.

nameValuePair :: String -> Any -> { name :: String; value :: Any; }

nameValuePair "some" 6
=> { name = "some"; value = 6; }

Apply a function to each element in an attribute set, creating a new attribute set.

mapAttrs :: (String -> Any -> Any) -> AttrSet -> AttrSet

mapAttrs (name: value: name + "-" + value)
   { x = "foo"; y = "bar"; }
=> { x = "x-foo"; y = "y-bar"; }

Like mapAttrs, but allows the name of each attribute to be changed in addition to the value. The applied function should return both the new name and value as a nameValuePair.

mapAttrs' :: (String -> Any -> { name :: String; value :: Any; }) -> AttrSet -> AttrSet

mapAttrs' (name: value: nameValuePair ("foo_" + name) ("bar-" + value))
   { x = "a"; y = "b"; }
=> { foo_x = "bar-a"; foo_y = "bar-b"; }

Call a function for each attribute in the given set and return the result in a list.

mapAttrsToList :: (String -> a -> b) -> AttrSet -> [b]

mapAttrsToList (name: value: name + value)
   { x = "a"; y = "b"; }
=> [ "xa" "yb" ]

Deconstruct an attrset to a list of name-value pairs as expected by builtins.listToAttrs. Each element of the resulting list is an attribute set with these attributes:

The following is always true:

builtins.listToAttrs (attrsToList attrs) == attrs

attrsToList (builtins.listToAttrs list) != list

attrsToList :: AttrSet -> [ { name :: String; value :: Any; } ]

attrsToList { foo = 1; bar = "asdf"; }
=> [ { name = "bar"; value = "asdf"; } { name = "foo"; value = 1; } ]

Like mapAttrs, except that it recursively applies itself to the leaf attributes of a potentially-nested attribute set: the second argument of the function will never be an attrset. Also, the first argument of the mapping function is a list of the attribute names that form the path to the leaf attribute.

For a function that gives you control over what counts as a leaf, see mapAttrsRecursiveCond.

mapAttrsRecursive (path: value: concatStringsSep "-" (path ++ [value]))
  { n = { a = "A"; m = { b = "B"; c = "C"; }; }; d = "D"; }

{ n = { a = "n-a-A"; m = { b = "n-m-b-B"; c = "n-m-c-C"; }; }; d = "d-D"; }

mapAttrsRecursive :: ([String] -> a -> b) -> AttrSet -> AttrSet

Like mapAttrsRecursive, but it takes an additional predicate that tells it whether to recurse into an attribute set. If the predicate returns false, mapAttrsRecursiveCond does not recurse, but instead applies the mapping function. If the predicate returns true, it does recurse, and does not apply the mapping function.

mapAttrsRecursiveCond
  (as: !(as ? "type" && as.type == "derivation"))
  (x: x.name)
  attrs

mapAttrsRecursiveCond :: (AttrSet -> Bool) -> ([String] -> a -> b) -> AttrSet -> AttrSet

Generate an attribute set by mapping a function over a list of attribute names.

genAttrs :: [ String ] -> (String -> Any) -> AttrSet

genAttrs [ "foo" "bar" ] (name: "x_" + name)
=> { foo = "x_foo"; bar = "x_bar"; }

Check whether the argument is a derivation. Any set with { type = "derivation"; } counts as a derivation.

isDerivation :: Any -> Bool

nixpkgs = import <nixpkgs> {}
isDerivation nixpkgs.ruby
=> true
isDerivation "foobar"
=> false

Converts a store path to a fake derivation.

toDerivation :: Path -> Derivation

If cond is true, return the attribute set as, otherwise an empty attribute set.

optionalAttrs :: Bool -> AttrSet -> AttrSet

optionalAttrs (true) { my = "set"; }
=> { my = "set"; }
optionalAttrs (false) { my = "set"; }
=> { }

Merge sets of attributes and use the function f to merge attributes values.

zipAttrsWithNames :: [ String ] -> (String -> [ Any ] -> Any) -> [ AttrSet ] -> AttrSet

zipAttrsWithNames ["a"] (name: vs: vs) [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; }

Merge sets of attributes and use the function f to merge attribute values. Like lib.attrsets.zipAttrsWithNames with all key names are passed for names.

Implementation note: Common names appear multiple times in the list of names, hopefully this does not affect the system because the maximal laziness avoid computing twice the same expression and listToAttrs does not care about duplicated attribute names.

zipAttrsWith :: (String -> [ Any ] -> Any) -> [ AttrSet ] -> AttrSet

zipAttrsWith (name: values: values) [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; b = ["z"]; }

Merge sets of attributes and combine each attribute value in to a list.

Like lib.attrsets.zipAttrsWith with (name: values: values) as the function.

zipAttrs :: [ AttrSet ] -> AttrSet

zipAttrs [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; b = ["z"]; }

Merge a list of attribute sets together using the // operator. In case of duplicate attributes, values from later list elements take precedence over earlier ones. The result is the same as foldl mergeAttrs { }, but the performance is better for large inputs. For n list elements, each with an attribute set containing m unique attributes, the complexity of this operation is O(nm log n).

mergeAttrsList :: [ Attrs ] -> Attrs

mergeAttrsList [ { a = 0; b = 1; } { c = 2; d = 3; } ]
=> { a = 0; b = 1; c = 2; d = 3; }
mergeAttrsList [ { a = 0; } { a = 1; } ]
=> { a = 1; }

Does the same as the update operator ‘//’ except that attributes are merged until the given predicate is verified. The predicate should accept 3 arguments which are the path to reach the attribute, a part of the first attribute set and a part of the second attribute set. When the predicate is satisfied, the value of the first attribute set is replaced by the value of the second attribute set.

recursiveUpdateUntil :: ( [ String ] -> AttrSet -> AttrSet -> Bool ) -> AttrSet -> AttrSet -> AttrSet

recursiveUpdateUntil (path: l: r: path == ["foo"]) {
##  # first attribute set
  foo.bar = 1;
  foo.baz = 2;
  bar = 3;
} {
##  #second attribute set
  foo.bar = 1;
  foo.quz = 2;
  baz = 4;
}

=> {
  foo.bar = 1; # 'foo.*' from the second set
  foo.quz = 2; #
  bar = 3;     # 'bar' from the first set
  baz = 4;     # 'baz' from the second set
}

A recursive variant of the update operator ‘//’. The recursion stops when one of the attribute values is not an attribute set, in which case the right hand side value takes precedence over the left hand side value.

recursiveUpdate :: AttrSet -> AttrSet -> AttrSet

recursiveUpdate {
  boot.loader.grub.enable = true;
  boot.loader.grub.device = "/dev/hda";
} {
  boot.loader.grub.device = "";
}

returns: {
  boot.loader.grub.enable = true;
  boot.loader.grub.device = "";
}

Recurse into every attribute set of the first argument and check that:

matchAttrs :: AttrSet -> AttrSet -> Bool

matchAttrs { cpu = {}; } { cpu = { bits = 64; }; }
=> true

Override only the attributes that are already present in the old set useful for deep-overriding.

overrideExisting :: AttrSet -> AttrSet -> AttrSet

overrideExisting {} { a = 1; }
=> {}
overrideExisting { b = 2; } { a = 1; }
=> { b = 2; }
overrideExisting { a = 3; b = 2; } { a = 1; }
=> { a = 1; b = 2; }

Turns a list of strings into a human-readable description of those strings represented as an attribute path. The result of this function is not intended to be machine-readable. Create a new attribute set with value set at the nested attribute location specified in attrPath.

showAttrPath :: [String] -> String

showAttrPath [ "foo" "10" "bar" ]
=> "foo.\"10\".bar"
showAttrPath []
=> "<root attribute path>"

Get a package output. If no output is found, fallback to .out and then to the default.

getOutput :: String -> Derivation -> String

getOutput "dev" pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"

Get a package’s bin output. If the output does not exist, fallback to .out and then to the default.

getBin :: Derivation -> String

getBin pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r"

Get a package’s lib output. If the output does not exist, fallback to .out and then to the default.

getLib :: Derivation -> String

getLib pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-lib"

Get a package’s dev output. If the output does not exist, fallback to .out and then to the default.

getDev :: Derivation -> String

getDev pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"

Get a package’s man output. If the output does not exist, fallback to .out and then to the default.

getMan :: Derivation -> String

getMan pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-man"

Pick the outputs of packages to place in buildInputs

chooseDevOutputs :: [Derivation] -> [String]

Make various Nix tools consider the contents of the resulting attribute set when looking for what to build, find, etc.

This function only affects a single attribute set; it does not apply itself recursively for nested attribute sets.

recurseIntoAttrs :: AttrSet -> AttrSet

{ pkgs ? import <nixpkgs> {} }:
{
  myTools = pkgs.lib.recurseIntoAttrs {
    inherit (pkgs) hello figlet;
  };
}

Undo the effect of recurseIntoAttrs.

dontRecurseIntoAttrs :: AttrSet -> AttrSet

unionOfDisjoint x y is equal to x // y // z where the attrnames in z are the intersection of the attrnames in x and y, and all values assert with an error message. This operator is commutative, unlike (//).

unionOfDisjoint :: AttrSet -> AttrSet -> AttrSet

Type: concatStrings :: [string] -> string

Concatenate a list of strings.

concatStrings ["foo" "bar"]
=> "foobar"

Located at lib/strings.nix:50 in <nixpkgs>.

Type: concatMapStrings :: (a -> string) -> [a] -> string

Map a function over a list and concatenate the resulting strings.

concatMapStrings (x: "a" + x) ["foo" "bar"]
=> "afooabar"

Located at lib/strings.nix:60 in <nixpkgs>.

Type: concatImapStrings :: (int -> a -> string) -> [a] -> string

Like concatMapStrings except that the f functions also gets the position as a parameter.

concatImapStrings (pos: x: "${toString pos}-${x}") ["foo" "bar"]
=> "1-foo2-bar"

Located at lib/strings.nix:71 in <nixpkgs>.

Type: intersperse :: a -> [a] -> [a]

Place an element between each element of a list

intersperse "/" ["usr" "local" "bin"]
=> ["usr" "/" "local" "/" "bin"].

Located at lib/strings.nix:81 in <nixpkgs>.

Type: concatStringsSep :: string -> [string] -> string

Concatenate a list of strings with a separator between each element

concatStringsSep "/" ["usr" "local" "bin"]
=> "usr/local/bin"

Located at lib/strings.nix:98 in <nixpkgs>.

Type: concatMapStringsSep :: string -> (a -> string) -> [a] -> string

Maps a function over a list of strings and then concatenates the result with the specified separator interspersed between elements.

concatMapStringsSep "-" (x: toUpper x)  ["foo" "bar" "baz"]
=> "FOO-BAR-BAZ"

Located at lib/strings.nix:110 in <nixpkgs>.

Type: concatIMapStringsSep :: string -> (int -> a -> string) -> [a] -> string

Same as concatMapStringsSep, but the mapping function additionally receives the position of its argument.

concatImapStringsSep "-" (pos: x: toString (x / pos)) [ 6 6 6 ]
=> "6-3-2"

Located at lib/strings.nix:127 in <nixpkgs>.

Type: concatLines :: [string] -> string

Concatenate a list of strings, adding a newline at the end of each one. Defined as concatMapStrings (s: s + "\n").

concatLines [ "foo" "bar" ]
=> "foo\nbar\n"

Located at lib/strings.nix:144 in <nixpkgs>.

Type: replicate :: int -> string -> string

Replicate a string n times, and concatenate the parts into a new string.

replicate 3 "v"
=> "vvv"
replicate 5 "hello"
=> "hellohellohellohellohello"

Located at lib/strings.nix:158 in <nixpkgs>.

Type: makeSearchPath :: string -> [string] -> string

Construct a Unix-style, colon-separated search path consisting of the given subDir appended to each of the given paths.

makeSearchPath "bin" ["/root" "/usr" "/usr/local"]
=> "/root/bin:/usr/bin:/usr/local/bin"
makeSearchPath "bin" [""]
=> "/bin"

Located at lib/strings.nix:171 in <nixpkgs>.

Type: string -> string -> [package] -> string

Construct a Unix-style search path by appending the given subDir to the specified output of each of the packages. If no output by the given name is found, fallback to .out and then to the default.

makeSearchPathOutput "dev" "bin" [ pkgs.openssl pkgs.zlib ]
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev/bin:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8/bin"

Located at lib/strings.nix:189 in <nixpkgs>.

Construct a library search path (such as RPATH) containing the libraries for a set of packages

makeLibraryPath [ "/usr" "/usr/local" ]
=> "/usr/lib:/usr/local/lib"
pkgs = import <nixpkgs> { }
makeLibraryPath [ pkgs.openssl pkgs.zlib ]
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r/lib:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8/lib"

Located at lib/strings.nix:207 in <nixpkgs>.

Construct an include search path (such as C_INCLUDE_PATH) containing the header files for a set of packages or paths.

makeIncludePath [ "/usr" "/usr/local" ]
=> "/usr/include:/usr/local/include"
pkgs = import <nixpkgs> { }
makeIncludePath [ pkgs.openssl pkgs.zlib ]
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev/include:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8-dev/include"

Located at lib/strings.nix:219 in <nixpkgs>.

Construct a binary search path (such as $PATH) containing the binaries for a set of packages.

makeBinPath ["/root" "/usr" "/usr/local"]
=> "/root/bin:/usr/bin:/usr/local/bin"

Located at lib/strings.nix:228 in <nixpkgs>.

Type: normalizePath :: string -> string

Normalize path, removing extraneous /s

normalizePath "/a//b///c/"
=> "/a/b/c/"

Located at lib/strings.nix:238 in <nixpkgs>.

Type: optionalString :: bool -> string -> string

Depending on the boolean `cond’, return either the given string or the empty string. Useful to concatenate against a bigger string.

optionalString true "some-string"
=> "some-string"
optionalString false "some-string"
=> ""

Located at lib/strings.nix:264 in <nixpkgs>.

Type: hasPrefix :: string -> string -> bool

Determine whether a string has given prefix.

hasPrefix "foo" "foobar"
=> true
hasPrefix "foo" "barfoo"
=> false

Located at lib/strings.nix:280 in <nixpkgs>.

Type: hasSuffix :: string -> string -> bool

Determine whether a string has given suffix.

hasSuffix "foo" "foobar"
=> false
hasSuffix "foo" "barfoo"
=> true

Located at lib/strings.nix:307 in <nixpkgs>.

Type: hasInfix :: string -> string -> bool

Determine whether a string contains the given infix

hasInfix "bc" "abcd"
=> true
hasInfix "ab" "abcd"
=> true
hasInfix "cd" "abcd"
=> true
hasInfix "foo" "abcd"
=> false

Located at lib/strings.nix:344 in <nixpkgs>.

Type: stringToCharacters :: string -> [string]

Convert a string to a list of characters (i.e. singleton strings). This allows you to, e.g., map a function over each character. However, note that this will likely be horribly inefficient; Nix is not a general purpose programming language. Complex string manipulations should, if appropriate, be done in a derivation. Also note that Nix treats strings as a list of bytes and thus doesn’t handle unicode.

stringToCharacters ""
=> [ ]
stringToCharacters "abc"
=> [ "a" "b" "c" ]
stringToCharacters "🦄"
=> [ "�" "�" "�" "�" ]

Located at lib/strings.nix:374 in <nixpkgs>.

Type: stringAsChars :: (string -> string) -> string -> string

Manipulate a string character by character and replace them by strings before concatenating the results.

stringAsChars (x: if x == "a" then "i" else x) "nax"
=> "nix"

Located at lib/strings.nix:386 in <nixpkgs>.

Type: charToInt :: string -> int

Convert char to ascii value, must be in printable range

charToInt "A"
=> 65
charToInt "("
=> 40

Located at lib/strings.nix:405 in <nixpkgs>.

Type: escape :: [string] -> string -> string

Escape occurrence of the elements of list in string by prefixing it with a backslash.

escape ["(" ")"] "(foo)"
=> "\\(foo\\)"

Located at lib/strings.nix:416 in <nixpkgs>.

Type: escapeC = [string] -> string -> string

Escape occurrence of the element of list in string by converting to its ASCII value and prefixing it with \x. Only works for printable ascii characters.

escapeC [" "] "foo bar"
=> "foo\\x20bar"

Located at lib/strings.nix:429 in <nixpkgs>.

Type: escapeURL :: string -> string

Escape the string so it can be safely placed inside a URL query.

escapeURL "foo/bar baz"
=> "foo%2Fbar%20baz"

Located at lib/strings.nix:440 in <nixpkgs>.

Type: escapeShellArg :: string -> string

Quote string to be used safely within the Bourne shell.

escapeShellArg "esc'ape\nme"
=> "'esc'\\''ape\nme'"

Located at lib/strings.nix:454 in <nixpkgs>.

Type: escapeShellArgs :: [string] -> string

Quote all arguments to be safely passed to the Bourne shell.

escapeShellArgs ["one" "two three" "four'five"]
=> "'one' 'two three' 'four'\\''five'"

Located at lib/strings.nix:464 in <nixpkgs>.

Type: string -> bool

Test whether the given name is a valid POSIX shell variable name.

isValidPosixName "foo_bar000"
=> true
isValidPosixName "0-bad.jpg"
=> false

Located at lib/strings.nix:476 in <nixpkgs>.

Type: string -> (string | listOf string | attrsOf string) -> string

Translate a Nix value into a shell variable declaration, with proper escaping.

The value can be a string (mapped to a regular variable), a list of strings (mapped to a Bash-style array) or an attribute set of strings (mapped to a Bash-style associative array). Note that “string” includes string-coercible values like paths or derivations.

Strings are translated into POSIX sh-compatible code; lists and attribute sets assume a shell that understands Bash syntax (e.g. Bash or ZSH).

''
  ${toShellVar "foo" "some string"}
  [[ "$foo" == "some string" ]]
''

Located at lib/strings.nix:496 in <nixpkgs>.

Type: attrsOf (string | listOf string | attrsOf string) -> string

Translate an attribute set into corresponding shell variable declarations using toShellVar.

let
  foo = "value";
  bar = foo;
in ''
  ${toShellVars { inherit foo bar; }}
  [[ "$foo" == "$bar" ]]
''

Located at lib/strings.nix:524 in <nixpkgs>.

Type: string -> string

Turn a string into a Nix expression representing that string

escapeNixString "hello\${}\n"
=> "\"hello\\\${}\\n\""

Located at lib/strings.nix:534 in <nixpkgs>.

Type: string -> string

Turn a string into an exact regular expression

escapeRegex "[^a-z]*"
=> "\\[\\^a-z]\\*"

Located at lib/strings.nix:544 in <nixpkgs>.

Type: string -> string

Quotes a string if it can’t be used as an identifier directly.

escapeNixIdentifier "hello"
=> "hello"
escapeNixIdentifier "0abc"
=> "\"0abc\""

Located at lib/strings.nix:556 in <nixpkgs>.

Type: string -> string

Escapes a string such that it is safe to include verbatim in an XML document.

escapeXML ''"test" 'test' < & >''
=> "&quot;test&quot; &apos;test&apos; &lt; &amp; &gt;"

Located at lib/strings.nix:570 in <nixpkgs>.

Type: toLower :: string -> string

Converts an ASCII string to lower-case.

toLower "HOME"
=> "home"

Located at lib/strings.nix:589 in <nixpkgs>.

Type: toUpper :: string -> string

Converts an ASCII string to upper-case.

toUpper "home"
=> "HOME"

Located at lib/strings.nix:599 in <nixpkgs>.

Appends string context from another string. This is an implementation detail of Nix and should be used carefully.

Strings in Nix carry an invisible context which is a list of strings representing store paths. If the string is later used in a derivation attribute, the derivation will properly populate the inputDrvs and inputSrcs.

pkgs = import <nixpkgs> { };
addContextFrom pkgs.coreutils "bar"
=> "bar"

Located at lib/strings.nix:614 in <nixpkgs>.

Cut a string with a separator and produces a list of strings which were separated by this separator.

splitString "." "foo.bar.baz"
=> [ "foo" "bar" "baz" ]
splitString "/" "/usr/local/bin"
=> [ "" "usr" "local" "bin" ]

Located at lib/strings.nix:625 in <nixpkgs>.

Type: string -> string -> string

Return a string without the specified prefix, if the prefix matches.

removePrefix "foo." "foo.bar.baz"
=> "bar.baz"
removePrefix "xxx" "foo.bar.baz"
=> "foo.bar.baz"

Located at lib/strings.nix:641 in <nixpkgs>.

Type: string -> string -> string

Return a string without the specified suffix, if the suffix matches.

removeSuffix "front" "homefront"
=> "home"
removeSuffix "xxx" "homefront"
=> "homefront"

Located at lib/strings.nix:674 in <nixpkgs>.

Return true if string v1 denotes a version older than v2.

versionOlder "1.1" "1.2"
=> true
versionOlder "1.1" "1.1"
=> false

Located at lib/strings.nix:705 in <nixpkgs>.

Return true if string v1 denotes a version equal to or newer than v2.

versionAtLeast "1.1" "1.0"
=> true
versionAtLeast "1.1" "1.1"
=> true
versionAtLeast "1.1" "1.2"
=> false

Located at lib/strings.nix:717 in <nixpkgs>.

This function takes an argument that’s either a derivation or a derivation’s “name” attribute and extracts the name part from that argument.

getName "youtube-dl-2016.01.01"
=> "youtube-dl"
getName pkgs.youtube-dl
=> "youtube-dl"

Located at lib/strings.nix:729 in <nixpkgs>.

This function takes an argument that’s either a derivation or a derivation’s “name” attribute and extracts the version part from that argument.

getVersion "youtube-dl-2016.01.01"
=> "2016.01.01"
getVersion pkgs.youtube-dl
=> "2016.01.01"

Located at lib/strings.nix:746 in <nixpkgs>.

Extract name with version from URL. Ask for separator which is supposed to start extension.

nameFromURL "https://nixos.org/releases/nix/nix-1.7/nix-1.7-x86_64-linux.tar.bz2" "-"
=> "nix"
nameFromURL "https://nixos.org/releases/nix/nix-1.7/nix-1.7-x86_64-linux.tar.bz2" "_"
=> "nix-1.7-x86"

Located at lib/strings.nix:762 in <nixpkgs>.

Type:

cmakeOptionType :: string -> string -> string -> string

@param feature The feature to be set
@param type The type of the feature to be set, as described in
            https://cmake.org/cmake/help/latest/command/set.html
            the possible values (case insensitive) are:
            BOOL FILEPATH PATH STRING INTERNAL
@param value The desired value

Create a “-D<feature>:<type>=<value>” string that can be passed to typical CMake invocations.

cmakeOptionType "string" "ENGINE" "sdl2"
=> "-DENGINE:STRING=sdl2"

Located at lib/strings.nix:785 in <nixpkgs>.

Type:

cmakeBool :: string -> bool -> string

@param condition The condition to be made true or false
@param flag The controlling flag of the condition

Create a -D<condition>={TRUE,FALSE} string that can be passed to typical CMake invocations.

cmakeBool "ENABLE_STATIC_LIBS" false
=> "-DENABLESTATIC_LIBS:BOOL=FALSE"

Located at lib/strings.nix:805 in <nixpkgs>.

Type:

cmakeFeature :: string -> string -> string

@param condition The condition to be made true or false
@param flag The controlling flag of the condition

Create a -D<feature>:STRING=<value> string that can be passed to typical CMake invocations. This is the most typical usage, so it deserves a special case.

cmakeFeature "MODULES" "badblock"
=> "-DMODULES:STRING=badblock"

Located at lib/strings.nix:823 in <nixpkgs>.

Type:

mesonOption :: string -> string -> string

@param feature The feature to be set
@param value The desired value

Create a -D<feature>=<value> string that can be passed to typical Meson invocations.

mesonOption "engine" "opengl"
=> "-Dengine=opengl"

Located at lib/strings.nix:840 in <nixpkgs>.

Type:

mesonBool :: string -> bool -> string

@param condition The condition to be made true or false
@param flag The controlling flag of the condition

Create a -D<condition>={true,false} string that can be passed to typical Meson invocations.

mesonBool "hardened" true
=> "-Dhardened=true"
mesonBool "static" false
=> "-Dstatic=false"

Located at lib/strings.nix:859 in <nixpkgs>.

Type:

mesonEnable :: string -> bool -> string

@param feature The feature to be enabled or disabled
@param flag The controlling flag

Create a -D<feature>={enabled,disabled} string that can be passed to typical Meson invocations.

mesonEnable "docs" true
=> "-Ddocs=enabled"
mesonEnable "savage" false
=> "-Dsavage=disabled"

Located at lib/strings.nix:878 in <nixpkgs>.

Create an --{enable,disable}-<feature> string that can be passed to standard GNU Autoconf scripts.

enableFeature true "shared"
=> "--enable-shared"
enableFeature false "shared"
=> "--disable-shared"

Located at lib/strings.nix:892 in <nixpkgs>.

Create an --{enable-<feature>=<value>,disable-<feature>} string that can be passed to standard GNU Autoconf scripts.

enableFeatureAs true "shared" "foo"
=> "--enable-shared=foo"
enableFeatureAs false "shared" (throw "ignored")
=> "--disable-shared"

Located at lib/strings.nix:906 in <nixpkgs>.

Create an --{with,without}-<feature> string that can be passed to standard GNU Autoconf scripts.

withFeature true "shared"
=> "--with-shared"
withFeature false "shared"
=> "--without-shared"

Located at lib/strings.nix:918 in <nixpkgs>.

Create an --{with-<feature>=<value>,without-<feature>} string that can be passed to standard GNU Autoconf scripts.

withFeatureAs true "shared" "foo"
=> "--with-shared=foo"
withFeatureAs false "shared" (throw "ignored")
=> "--without-shared"

Located at lib/strings.nix:931 in <nixpkgs>.

Type: fixedWidthString :: int -> string -> string -> string

Create a fixed width string with additional prefix to match required width.

This function will fail if the input string is longer than the requested length.

fixedWidthString 5 "0" (toString 15)
=> "00015"

Located at lib/strings.nix:946 in <nixpkgs>.

Format a number adding leading zeroes up to fixed width.

fixedWidthNumber 5 15
=> "00015"

Located at lib/strings.nix:963 in <nixpkgs>.

Convert a float to a string, but emit a warning when precision is lost during the conversion

floatToString 0.000001
=> "0.000001"
floatToString 0.0000001
=> trace: warning: Imprecise conversion from float to string 0.000000
   "0.000000"

Located at lib/strings.nix:975 in <nixpkgs>.

Soft-deprecated function. While the original implementation is available as isConvertibleWithToString, consider using isStringLike instead, if suitable.

Located at lib/strings.nix:983 in <nixpkgs>.

Check whether a list or other value can be passed to toString.

Many types of value are coercible to string this way, including int, float, null, bool, list of similarly coercible values.

Located at lib/strings.nix:992 in <nixpkgs>.

Check whether a value can be coerced to a string. The value must be a string, path, or attribute set.

String-like values can be used without explicit conversion in string interpolations and in most functions that expect a string.

Located at lib/strings.nix:1005 in <nixpkgs>.

Check whether a value is a store path.

isStorePath "/nix/store/d945ibfx9x185xf04b890y4f9g3cbb63-python-2.7.11/bin/python"
=> false
isStorePath "/nix/store/d945ibfx9x185xf04b890y4f9g3cbb63-python-2.7.11"
=> true
isStorePath pkgs.python
=> true
isStorePath [] || isStorePath 42 || isStorePath {} || …
=> false

Located at lib/strings.nix:1023 in <nixpkgs>.

Type: string -> int

Parse a string as an int. Does not support parsing of integers with preceding zero due to ambiguity between zero-padded and octal numbers. See toIntBase10.

toInt "1337"
=> 1337

toInt "-4"
=> -4

toInt " 123 "
=> 123

toInt "00024"
=> error: Ambiguity in interpretation of 00024 between octal and zero padded integer.

toInt "3.14"
=> error: floating point JSON numbers are not supported

Located at lib/strings.nix:1053 in <nixpkgs>.

Type: string -> int

Parse a string as a base 10 int. This supports parsing of zero-padded integers.

toIntBase10 "1337"
=> 1337

toIntBase10 "-4"
=> -4

toIntBase10 " 123 "
=> 123

toIntBase10 "00024"
=> 24

toIntBase10 "3.14"
=> error: floating point JSON numbers are not supported

Located at lib/strings.nix:1106 in <nixpkgs>.

Read a list of paths from file, relative to the rootPath. Lines beginning with # are treated as comments and ignored. Whitespace is significant.

NOTE: This function is not performant and should be avoided.

readPathsFromFile /prefix
  ./pkgs/development/libraries/qt-5/5.4/qtbase/series
=> [ "/prefix/dlopen-resolv.patch" "/prefix/tzdir.patch"
     "/prefix/dlopen-libXcursor.patch" "/prefix/dlopen-openssl.patch"
     "/prefix/dlopen-dbus.patch" "/prefix/xdg-config-dirs.patch"
     "/prefix/nix-profiles-library-paths.patch"
     "/prefix/compose-search-path.patch" ]

Located at lib/strings.nix:1154 in <nixpkgs>.

Type: fileContents :: path -> string

Read the contents of a file removing the trailing \n

$ echo "1.0" > ./version

fileContents ./version
=> "1.0"

Located at lib/strings.nix:1174 in <nixpkgs>.

Type: sanitizeDerivationName :: String -> String

Creates a valid derivation name from a potentially invalid one.

sanitizeDerivationName "../hello.bar # foo"
=> "-hello.bar-foo"
sanitizeDerivationName ""
=> "unknown"
sanitizeDerivationName pkgs.hello
=> "-nix-store-2g75chlbpxlrqn15zlby2dfh8hr9qwbk-hello-2.10"

Located at lib/strings.nix:1189 in <nixpkgs>.

Type: levenshtein :: string -> string -> int

Computes the Levenshtein distance between two strings. Complexity O(n*m) where n and m are the lengths of the strings. Algorithm adjusted from https://stackoverflow.com/a/9750974/6605742

levenshtein "foo" "foo"
=> 0
levenshtein "book" "hook"
=> 1
levenshtein "hello" "Heyo"
=> 3

Located at lib/strings.nix:1228 in <nixpkgs>.

Returns the length of the prefix common to both strings.

Located at lib/strings.nix:1249 in <nixpkgs>.

Returns the length of the suffix common to both strings.

Located at lib/strings.nix:1257 in <nixpkgs>.

Type: levenshteinAtMost :: int -> string -> string -> bool

Returns whether the levenshtein distance between two strings is at most some value Complexity is O(min(n,m)) for k <= 2 and O(n*m) otherwise

levenshteinAtMost 0 "foo" "foo"
=> true
levenshteinAtMost 1 "foo" "boa"
=> false
levenshteinAtMost 2 "foo" "boa"
=> true
levenshteinAtMost 2 "This is a sentence" "this is a sentense."
=> false
levenshteinAtMost 3 "This is a sentence" "this is a sentense."
=> true

Located at lib/strings.nix:1281 in <nixpkgs>.

Break a version string into its component parts.

splitVersion "1.2.3"
=> ["1" "2" "3"]

Located at lib/versions.nix:12 in <nixpkgs>.

Get the major version string from a string.

major "1.2.3"
=> "1"

Located at lib/versions.nix:20 in <nixpkgs>.

Get the minor version string from a string.

minor "1.2.3"
=> "2"

Located at lib/versions.nix:28 in <nixpkgs>.

Get the patch version string from a string.

patch "1.2.3"
=> "3"

Located at lib/versions.nix:36 in <nixpkgs>.

Get string of the first two parts (major and minor) of a version string.

majorMinor "1.2.3"
=> "1.2"

Located at lib/versions.nix:45 in <nixpkgs>.

Pad a version string with zeros to match the given number of components.

pad 3 "1.2"
=> "1.2.0"
pad 3 "1.3-rc1"
=> "1.3.0-rc1"
pad 3 "1.2.3.4"
=> "1.2.3"

Located at lib/versions.nix:59 in <nixpkgs>.

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

id :: a -> a

The constant function

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

const :: a -> b -> a

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

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

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

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.

Concatenate two lists

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

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

boolean “or”

boolean “and”

boolean “exclusive or”

bitwise “not”

Located at lib/trivial.nix:223 in <nixpkgs>.

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!).

boolToString :: bool -> string

Merge two attribute sets shallowly, right side trumps left

mergeAttrs :: attrs -> attrs -> attrs

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

Flip the order of the arguments of a binary function.

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

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

Apply function if the supplied argument is non-null.

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

Returns the current full nixpkgs version number.

Located at lib/trivial.nix:360 in <nixpkgs>.

Returns the current nixpkgs release number as string.

Located at lib/trivial.nix:365 in <nixpkgs>.

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.

Located at lib/trivial.nix:380 in <nixpkgs>.

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

Located at lib/trivial.nix:406 in <nixpkgs>.

Returns the current nixpkgs version suffix as string.

Located at lib/trivial.nix:411 in <nixpkgs>.

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

revisionWithDefault :: string -> string

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

inNixShell :: 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.

inPureEvalMode :: bool

Return minimum of two numbers.

Return maximum of two numbers.

Integer modulus

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

C-style comparisons

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

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.

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

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

Reads a JSON file.

importJSON :: path -> any

Reads a TOML file.

importTOML :: path -> any

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

string -> a -> a

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

bool -> string -> a -> a

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

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.

bool -> string -> a -> a

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

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

bool -> string -> 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.

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

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

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.

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.

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