From 69920dafbffcd757acff23f659263ec4b952a017 Mon Sep 17 00:00:00 2001
From: danbst <abcz2.uprola@gmail.com>
Date: Sun, 14 Jul 2019 13:17:49 +0300
Subject: [PATCH] lib: introduce `foreach` = flip map

The main purpose is to bring attention to `flip map`, which improves
code readablity. It is useful when ad-hoc anonymous function
grows two or more lines in `map` application:

```
      map (lcfg:
        let port = lcfg.port;
            portStr = if port != defaultPort then ":${toString port}" else "";
            scheme = if cfg.enableSSL then "https" else "http";
        in "${scheme}://cfg.hostName${portStr}"
      ) (getListen cfg);
```
Compare this to `foreach`-style:
```
      foreach (getListen cfg) (lcfg:
        let port = lcfg.port;
            portStr = if port != defaultPort then ":${toString port}" else "";
            scheme = if cfg.enableSSL then "https" else "http";
        in "${scheme}://cfg.hostName${portStr}"
      );
```
This is similar to Haskell's `for` (http://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Traversable.html#v:for)
---
 lib/default.nix |  2 +-
 lib/lists.nix   | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/lib/default.nix b/lib/default.nix
index 3efaaf0f8f9..f876c57e25c 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -71,7 +71,7 @@ let
       zipAttrsWithNames zipAttrsWith zipAttrs recursiveUpdateUntil
       recursiveUpdate matchAttrs overrideExisting getOutput getBin
       getLib getDev chooseDevOutputs zipWithNames zip;
-    inherit (lists) singleton foldr fold foldl foldl' imap0 imap1
+    inherit (lists) singleton foreach foldr fold foldl foldl' imap0 imap1
       concatMap flatten remove findSingle findFirst any all count
       optional optionals toList range partition zipListsWith zipLists
       reverseList listDfs toposort sort naturalSort compareLists take
diff --git a/lib/lists.nix b/lib/lists.nix
index 30d87ece664..f9953720ee1 100644
--- a/lib/lists.nix
+++ b/lib/lists.nix
@@ -21,6 +21,19 @@ rec {
   */
   singleton = x: [x];
 
+  /*  Apply the function to each element in the list. Same as `map`, but arguments
+      flipped.
+
+      Type: foreach :: [a] -> (a -> b) -> [b]
+
+      Example:
+        foreach [ 1 2 ] (x:
+          toString x
+        )
+        => [ "1" "2" ]
+  */
+  foreach = xs: f: map f xs;
+
   /* “right fold” a binary function `op` between successive elements of
      `list` with `nul' as the starting value, i.e.,
      `foldr op nul [x_1 x_2 ... x_n] == op x_1 (op x_2 ... (op x_n nul))`.