nixpkgs/lib/trivial.nix

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Nix
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rec {
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/* The identity function
For when you need a function that does nothing.
Type: id :: a -> a
*/
id = x: x;
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/* The constant function
Ignores the second argument.
Or: Construct a function that always returns a static value.
Type: const :: a -> b -> a
Example:
let f = const 5; in f 10
=> 5
*/
const = x: y: x;
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## Named versions corresponding to some builtin operators.
/* Concat two strings */
concat = x: y: x ++ y;
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/* boolean or */
or = x: y: x || y;
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/* boolean and */
and = x: y: x && y;
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/* Convert a boolean to a string.
Note that toString on a bool returns "1" and "".
*/
boolToString = b: if b then "true" else "false";
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/* Merge two attribute sets shallowly, right side trumps left
Example:
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mergeAttrs { a = 1; b = 2; } { b = 3; c = 4; }
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=> { a = 1; b = 3; c = 4; }
*/
mergeAttrs = x: y: x // y;
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# Flip the order of the arguments of a binary function.
flip = f: a: b: f b a;
# Apply function if argument is non-null
mapNullable = f: a: if isNull a then a else f a;
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# Pull in some builtins not included elsewhere.
inherit (builtins)
pathExists readFile isBool isFunction
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isInt add sub lessThan
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seq deepSeq genericClosure;
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inherit (import ./strings.nix) fileContents;
# Return the Nixpkgs version number.
nixpkgsVersion =
let suffixFile = ../.version-suffix; in
fileContents ../.version
+ (if pathExists suffixFile then fileContents suffixFile else "pre-git");
# Whether we're being called by nix-shell.
inNixShell = builtins.getEnv "IN_NIX_SHELL" != "";
# Return minimum/maximum of two numbers.
min = x: y: if x < y then x else y;
max = x: y: if x > y then x else y;
/* Reads a JSON file. */
importJSON = path:
builtins.fromJSON (builtins.readFile path);
/* 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;
}
TODO: figure out a clever way to integrate location information from
something like __unsafeGetAttrPos.
*/
warn = msg: builtins.trace "WARNING: ${msg}";
info = msg: builtins.trace "INFO: ${msg}";
}