each{,Default}System{,Map}: accept binary functions

README.md

@@ -83,6 +83,16 @@ eachSystem allSystems (system: { hello = 42; })
 }
 ```
 
+If you pass a function rather than an attribute set after the `system` argument,
+it will receive as its argument the final attribute set. For example:
+
+```nix
+eachSystem [ system.x86_64-linux ] (system: self: { a = 2; b = self.a + 3; })
+```
+
+`self` is not that useful in this case, as it can be replaced with the `rec`
+keyword but there can be situations where it is preferred.
+
 ### `eachDefaultSystem :: (<system> -> attrs)`
 
 `eachSystem` pre-populated with `defaultSystems`.

lib.nix

@@ -33,7 +33,7 @@ let
       # Merge together the outputs for all systems.
       op = attrs: system:
         let
-          ret = f system;
+          ret = maybeFix (f system);
           op = attrs: key: attrs //
               {
                 ${key} = (attrs.${key} or { })
@@ -50,7 +50,8 @@ let
   eachDefaultSystemMap = eachSystemMap defaultSystems;
 
   # Builds a map from <attr>=value to <system>.<attr> = value.
-  eachSystemMap = systems: f: builtins.listToAttrs (builtins.map (system: { name = system; value = f system; }) systems);
+  eachSystemMap = systems: f:
+    builtins.listToAttrs (builtins.map (system: { name = system; value = maybeFix (f system); }) systems);
 
   # Nix flakes insists on having a flat attribute set of derivations in
   # various places like the `packages` and `checks` attributes.
@@ -201,5 +202,11 @@ let
       system
       ;
   };
+
+  maybeFix = arg:
+    let
+      fix = f: let x = f x; in x;
+    in
+    if builtins.isFunction arg then fix arg else arg;
 in
 lib