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>

1 files changed, 25 insertions, 0 deletions

diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index e5d76d4e57b..b064faa1e1b 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -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;