diff --git a/Main.hs b/Main.hs
new file mode 100644
index 0000000..451e2d1
--- /dev/null
+++ b/Main.hs
@@ -0,0 +1,73 @@
+-- TODO: qualified imports
+-- TODO: format code
+
+import Options.Applicative
+import Control.Monad
+import Data.Semigroup ((<>))
+import System.Directory
+import System.FilePath
+
+fileFetchNix :: FilePath
+fileFetchNix = "nix" </> "fetch.nix"
+
+fileFetchNixContent :: String
+fileFetchNixContent = unlines
+  [
+
+
+  ]
+
+fileVersionsJson :: FilePath
+fileVersionsJson = "nix" </> "versions.json"
+
+fileVersionsJsonContent :: String
+fileVersionsJsonContent = unlines
+  [
+
+  ]
+
+cmdInit :: IO ()
+cmdInit = do
+    putStrLn "Creating directory nix (if it doesn't exist)"
+    createDirectoryIfMissing True "nix"
+
+    putStrLn $ "Creating file " <> fileFetchNix <> " (if it doesn't exist)"
+    fileFetchNixExists <- doesFileExist fileFetchNix
+
+    if fileFetchNixExists
+    then do
+      putStrLn $ "Not writing " <> fileFetchNix
+      putStrLn "(file exists)"
+    else do
+      putStrLn $ "Writing " <> fileFetchNix
+      writeFile fileFetchNix fileFetchNixContent
+
+    putStrLn $ "Creating file " <> fileVersionsJson <> " (if it doesn't exist)"
+    fileVersionsJsonExists <- doesFileExist fileVersionsJson
+
+    if fileVersionsJsonExists
+    then do
+      putStrLn $ "Not writing " <> fileVersionsJson
+      putStrLn "(file exists)"
+    else do
+      putStrLn $ "Writing " <> fileVersionsJson
+      writeFile fileVersionsJson fileVersionsJsonContent
+
+cmdAdd :: IO ()
+cmdAdd = putStrLn "add"
+
+cmdShow :: IO ()
+cmdShow = putStrLn "show"
+
+cmdUpdate :: IO ()
+cmdUpdate = putStrLn "update"
+
+opts :: Parser (IO ())
+opts = subparser (
+    command "init" (info (pure cmdInit) idm) <>
+    command "add"  (info (pure cmdAdd) idm) <>
+    command "show"  (info (pure cmdAdd) idm) <>
+    command "update"  (info (pure cmdAdd) idm) )
+
+main :: IO ()
+main = join $ execParser (info opts idm)
diff --git a/README.md b/README.md
index a502877..09fab14 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,17 @@
 
 A tool for dealing with third-party packages in [Nix].
 
+## Building
+
+Inside the provided nix shell:
+
+``` bash
+$ # GHCi:
+$ snack ghci -p package.yaml
+$ # actual build:
+$ snack build -p package.yaml
+```
+
 ## Usage
 
 ### Global options
@@ -39,11 +50,11 @@ in pkgs.hello
 
 #### show
 
-`[--branch] [--rev] [--owner] [--repo] <p1> <p2>`... if no attribute
-  (br, rev, own, repo) is given, all attributes are shown for `<packages>`.
-  Otherwise the specified attributes are shown. If no package is specified:
-  `<packages> = <all packages>`, otherwise `<packages>` is set to the specified
-  packages.
+`[--branch] [--rev] [--owner] [--repo] [--attribute <attribute>] <p1> <p2>`...
+  if no attribute (br, rev, ...) is given, all attributes are shown for
+  `<packages>`.  Otherwise the specified attributes are shown. If no package is
+  specified: `<packages> = <all packages>`, otherwise `<packages>` is set to
+  the specified packages.
 
 #### update
 
@@ -69,9 +80,10 @@ in pkgs.hello
 }
 ```
 
-* `--branch`: specifies `<branch>`
+* `--branch`: specifies `<branch>` (default: master)
 * `--username <username>`: then `let <repo> = <package>`
 * `--gitlab`: use gitlab instead of GitHub
+* `--attribute <attribute> <value>`: sets `<attribute>` to `<value>`
 
 **NOTE**: should the URLs be used instead? or more simply, how do we differentiate between Gitlab/GitHub?
 
diff --git a/default.nix b/default.nix
new file mode 100644
index 0000000..e61aa12
--- /dev/null
+++ b/default.nix
@@ -0,0 +1,17 @@
+with { fetch = import ./nix/fetch.nix; };
+let
+
+  # The (pinned) Nixpkgs where the original packages are sourced from
+  pkgs = import fetch.nixpkgs {};
+
+  # The list of packages to be installed
+  shellPackages = [ snack-exe ];
+
+  snack-exe = (import fetch.snack).snack-exe;
+  snack-lib = (import fetch.snack).snack-lib;
+in
+  if pkgs.lib.inNixShell
+  then pkgs.mkShell
+    { buildInputs = shellPackages;
+    }
+  else snack-lib.inferHPackBuild ./package.yaml
diff --git a/nix/fetch.nix b/nix/fetch.nix
new file mode 100644
index 0000000..7da0c48
--- /dev/null
+++ b/nix/fetch.nix
@@ -0,0 +1,23 @@
+# A record, from name to path, of the third-party packages
+let
+  versions = builtins.fromJSON (builtins.readFile ./versions.json);
+  fetchTarball =
+    # fetchTarball version that is compatible between all the versions of
+    # Nix
+    { url, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchTarball;
+    in
+      if lessThan nixVersion "1.12" then
+        fetchTarball { inherit url; }
+      else
+        fetchTarball attrs;
+in
+  builtins.mapAttrs (_: spec:
+      fetchTarball {
+        url =
+          with spec;
+          "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
+        sha256 = spec.sha256;
+      }
+    ) versions
diff --git a/nix/versions.json b/nix/versions.json
new file mode 100644
index 0000000..5a95232
--- /dev/null
+++ b/nix/versions.json
@@ -0,0 +1,16 @@
+{
+  "nixpkgs": {
+    "owner": "NixOS",
+    "repo": "nixpkgs-channels",
+    "branch": "nixos-18.09",
+    "rev": "c2950341d038995bf46a7b72db961bb3d3e9ac12",
+    "sha256": "1adpvp20b3zwxchnvkag8d0m7qlnwxv3l9z1lg96kr5ffwk37prv"
+  },
+  "snack": {
+    "owner": "nmattia",
+    "repo": "snack",
+    "branch": "master",
+    "rev": "9754f4120d28ce25888a608606deddef9f490654",
+    "sha256": "1a2gcap2z41vk3d7cqydj880lwcpxljd49w6srb3gplciipha6zv"
+  }
+}
diff --git a/package.yaml b/package.yaml
new file mode 100644
index 0000000..c420a62
--- /dev/null
+++ b/package.yaml
@@ -0,0 +1,9 @@
+name: niv
+
+executable:
+    source-dirs: . # remove when https://github.com/nmattia/snack/pull/96 is merged
+    main: Main.hs
+    dependencies:
+        - directory
+        - filepath
+        - optparse-applicative