Continue moving heavy-lifting into library

lib/flake_env__util.re

@@ -1,10 +1,60 @@
 open Core;
 module Unix = Core_unix;
 
-let nix = (args) => {
-  let stdout_chan = Unix.open_process_in(
-    "nix --extra-experimental-features \"nix-command flakes\" " ++ (args |> String.concat(~sep=" ")));
+let run_process = (name, args) => {
+  /*** Run a process [name] with args [args], returning (exit_code, stdout text) */
+  let stdout_chan =
+    Unix.open_process_in(name ++ " " ++ (args |> String.concat(~sep=" ")));
   let stdout_content = stdout_chan |> In_channel.input_all;
   let exit_code = Unix.close_process_in(stdout_chan);
-  (exit_code, stdout_content)
-}
+  (exit_code, stdout_content);
+};
+
+let nix = args =>
+  run_process(
+    "nix",
+    ["--extra-experimental-features", "\"nix-command flakes\" ", ...args],
+  );
+
+let hash_files = filenames => {
+  /*** Hash all entries in [filenames], returning a hex-encoded string of the hash of their contents */
+  let ctx = Sha1.init();
+  let () =
+    filenames
+    |> Array.filter(~f=f =>
+         switch (Sys_unix.file_exists(f)) {
+         | `Yes => true
+         | _ => false
+         }
+       )
+    |> Array.iter(~f=f => {
+         f
+         |> In_channel.create
+         |> In_channel.input_all
+         |> Sha1.update_string(ctx)
+       });
+  Sha1.finalize(ctx) |> Sha1.to_hex;
+};
+
+let rec rmrf = path => {
+  switch (Unix.lstat(path).st_kind) {
+  | exception (Unix.Unix_error(_, _, _)) => ()
+  | S_REG | S_LNK => Unix.unlink(path)
+  | S_DIR =>
+    Sys_unix.readdir(path)
+    |> Array.iter(~f=name => rmrf(Filename.concat(path, name)));
+    Unix.rmdir(path);
+  | _ => Printf.eprintf("Unsupported file type (Chr or Block device, FIFO, or Socket)\n")
+  };
+};
+
+let get_args = argv => {
+  switch (Array.length(argv)) {
+  | x when x >= 3 =>
+    let layout_directory = argv[1];
+    let flake_specifier = argv[2];
+    let other_args = snd(List.split_n(List.of_array(argv), 3));
+    Ok((layout_directory, flake_specifier, other_args));
+  | _ => Error()
+  };
+};