lib/trivial: add pipe function

`pipe` is a useful operator for creating pipelines of functions.

It works around the usual problem of e.g. string operations becoming
deeply nested functions.

In principle, there are four different ways this function could be
written:

pipe val [ f1 .. fn ]
pipe val [ fn .. f1 ]
compose [ f1 .. fn ] val
compose [ fn .. f1 ] val

The third and fourth form mirror composition of functions, they would
be the same as e.g. `(f1 << f2 << f3 .. << fn) val`.
However, it is not clear which direction the list should have (as one
can see in the second form, which is the most absurd.

In order not to confuse users, we decide for the most “intuitive”
form, which mirrors the way unix pipes work (thus the name `pipe`).
The flow of data goes from left to right.

Co-Authored-By: Silvan Mosberger <infinisil@icloud.com>
This commit is contained in:
Profpatsch 2019-10-18 17:57:33 +02:00
parent 360e57a567
commit 8252861507
3 changed files with 64 additions and 2 deletions

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@ -57,8 +57,8 @@ let
hasAttr head isAttrs isBool isInt isList isString length
lessThan listToAttrs pathExists readFile replaceStrings seq
stringLength sub substring tail;
inherit (trivial) id const concat or and bitAnd bitOr bitXor bitNot
boolToString mergeAttrs flip mapNullable inNixShell min max
inherit (trivial) id const pipe concat or and bitAnd bitOr bitXor
bitNot boolToString mergeAttrs flip mapNullable inNixShell min max
importJSON warn info showWarnings nixpkgsVersion version mod compare
splitByAndCompare functionArgs setFunctionArgs isFunction;
inherit (fixedPoints) fix fix' converge extends composeExtensions

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@ -18,6 +18,31 @@ runTests {
expected = 2;
};
testPipe = {
expr = pipe 2 [
(x: x + 2) # 2 + 2 = 4
(x: x * 2) # 4 * 2 = 8
];
expected = 8;
};
testPipeEmpty = {
expr = pipe 2 [];
expected = 2;
};
testPipeStrings = {
expr = pipe [ 3 4 ] [
(map toString)
(map (s: s + "\n"))
concatStrings
];
expected = ''
3
4
'';
};
/*
testOr = {
expr = or true false;

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@ -29,6 +29,43 @@ rec {
# Value to ignore
y: x;
/* Pipes a value through a list of functions, left to right.
Type: pipe :: a -> [<functions>] -> <return type of last function>
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.
*/
pipe = val: functions:
let reverseApply = x: f: f x;
in builtins.foldl' reverseApply val functions;
/* note please dont 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, its obvious that it
goes first-to-last. With `compose`, not so much.
*/
## Named versions corresponding to some builtin operators.