Initial version

default.nix

@@ -0,0 +1,82 @@
+# Compatability function to allow flakes to be used by
+# non-flake-enabled Nix versions. Given a source tree containing a
+# 'flake.nix' and 'flake.lock' file, it fetches the flake inputs and
+# calls the flake's 'outputs' function. It then returns an attrset
+# containing 'defaultNix' (to be used in 'default.nix'), 'shellNix'
+# (to be used in 'shell.nix').
+
+{ src }:
+
+let
+
+  lockFile = builtins.fromJSON (builtins.readFile (src + "/flake.lock"));
+
+  fetchTree =
+    { info, inputs, original, locked }:
+    if locked.type == "github" then
+      { outPath = fetchTarball "https://api.github.com/repos/${locked.owner}/${locked.repo}/tarball/${locked.rev}";
+        rev = locked.rev;
+        shortRev = builtins.substring 0 7 locked.rev;
+        lastModified = formatSecondsSinceEpoch info.lastModified;
+        narHash = info.narHash;
+      }
+    else
+      # FIXME: add Git, Mercurial, tarball inputs.
+      throw "flake input has unsupported input type '${locked.type}'";
+
+  callFlake = flakeSrc: locks:
+    let
+      flake = import (flakeSrc + "/flake.nix");
+
+      inputs = builtins.mapAttrs (n: v: callFlake (fetchTree v) v.inputs) locks;
+
+      outputs = flakeSrc // (flake.outputs (inputs // {self = outputs;}));
+    in
+      assert flake.edition == 201909;
+      outputs;
+
+  src' =
+    (if src ? outPath then src else { outPath = src; })
+    // { lastModified = formatSecondsSinceEpoch 0; };
+
+  # Format number of seconds in the Unix epoch as %Y%m%d%H%M%S.
+  formatSecondsSinceEpoch = t:
+    let
+      rem = x: y: x - x / y * y;
+      days = t / 86400;
+      secondsInDay = rem t 86400;
+      hours = secondsInDay / 3600;
+      minutes = (rem secondsInDay 3600) / 60;
+      seconds = rem t 60;
+
+      # Courtesy of https://stackoverflow.com/a/32158604.
+      z = days + 719468;
+      era = (if z >= 0 then z else z - 146096) / 146097;
+      doe = z - era * 146097;
+      yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
+      y = yoe + era * 400;
+      doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
+      mp = (5 * doy + 2) / 153;
+      d = doy - (153 * mp + 2) / 5 + 1;
+      m = mp + (if mp < 10 then 3 else -9);
+      y' = y + (if m <= 2 then 1 else 0);
+
+      pad = s: if builtins.stringLength s < 2 then "0" + s else s;
+    in "${toString y'}${pad (toString m)}${pad (toString d)}${pad (toString hours)}${pad (toString minutes)}${pad (toString seconds)}";
+
+  result = callFlake src' (lockFile.inputs);
+
+  system = builtins.currentSystem or "unknown-system";
+
+in
+  assert lockFile.version == 4;
+
+  rec {
+    defaultNix =
+      result
+      // (if result ? defaultPackage.${system} then { default = result.defaultPackage.${system}; } else {});
+
+    shellNix =
+      defaultNix
+      // (if result ? devShell.${system} then { default = result.devShell.${system}; } else {});
+  }