diff --git a/README.md b/README.md
index f59dac9..9b46702 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,40 @@
 # flake-utils
-Pure Nix flake utility functions
+
+**STATUS: WIP**
+
+Pure Nix flake utility functions.
+
+The goal of this project is to build a collection of pure Nix functions that don't
+depend on nixpkgs, and that are useful in the context of writing other Nix
+flakes.
+
+## Usage
+
+`flake.nix`
+```nix
+{
+  edition = 201909;
+  description = "My flake";
+  inputs = {
+    utils = { type = "github"; owner = "numtide"; repo = "flake-utils"; };
+  };
+  outputs = { self, nixpkgs, utils }:
+    utils.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system}; in
+      rec {
+        packages = {
+          my-app = pkgs.callPackage ./my-app.nix {};
+        };
+
+        defaultPackage = package.my-app;
+
+        apps = {
+          my-app = flake.mkApp packages.my-app;
+        };
+
+        defaultApp = apps.my-app;
+      };
+    );
+}
+```
+
diff --git a/default.nix b/default.nix
index c79dea3..122cef6 100644
--- a/default.nix
+++ b/default.nix
@@ -1,28 +1,41 @@
-rec {
+let
   # copied from <nixpkgs/lib>
   genAttrs = names: f:
     builtins.listToAttrs (map (n: { name = n; value = f n; }) names);
 
+  mapAttrsToList = f: attrs:
+    map (name: f name attrs.${name}) (builtins.attrNames attrs);
+
   # The list of systems supported by nixpkgs and hydra
-  supportedSystems = [
+  defaultSystems = [
     "aarch64-linux"
     "i686-linux"
     "x86_64-darwin"
     "x86_64-linux"
   ];
 
-  # Returns an attribute set with all the supported systems as keys and the
-  # output of the passed function with each system passed to it.
+  # eachSystem using defaultSystems
+  eachDefaultSystem = eachSystem defaultSystems;
+
+  # Builds a map from <attr>=value to <system>.<attr>=value for each system.
   #
-  # This is useful in the flake outputs because the outputs return static sets
-  # that map to the different systems.
   #
-  # Example:
-  #   forAllSupported (x: null)
-  #   > { aarch64-linux = null; i686-linux = null; x86_64-darwin = null;
-  #   > x86_64-linux = null; }
-  # (system -> attrs) -> attrs
-  forAllSupported = genAttrs supportedSystems;
+  eachSystem = systems: f:
+    let
+      op = attrs: system:
+        let
+          ret = f system;
+          opt = attrs: key:
+            attrs //
+            {
+              ${key} = (attrs.${key} or {}) // { ${system} = ret.${key}; };
+            }
+            ;
+        in
+        builtins.foldl' op attrs (builtins.attrNames ret);
+    in
+    builtins.foldl' op {} systems
+    ;
 
   # Returns the structure used by `nix app`
   mkApp =
@@ -34,4 +47,12 @@ rec {
       type = "app";
       program = "${drv}${exePath}";
     };
+in
+{
+  inherit
+    defaultSystems
+    eachDefaultSystem
+    eachSystem
+    mkApp
+    ;
 }
diff --git a/example/flake.nix b/example/flake.nix
new file mode 100644
index 0000000..d2150e5
--- /dev/null
+++ b/example/flake.nix
@@ -0,0 +1,20 @@
+{
+  description = "Flake utils demo";
+  edition = 201909;
+
+  inputs.utils = {
+    type = "git";
+    uri = "file:///home/zimbatm/go/src/github.com/zimbatm/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, utils }:
+    utils.lib.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system}; in
+      rec {
+        packages.hello = pkgs.hello;
+        defaultPackage = packages.hello;
+        apps.hello = utils.lib.mkApp packages.hello;
+        defaultApp = apps.hello;
+      }
+    );
+}