Add flake input tracking

and restructure the whole codebase!

Not fully tested due to #4; theoretically resolves #1

lib/flake_env__versions.re

@@ -0,0 +1,82 @@
+open Core;
+module Unix = Core_unix;
+
+type t = {
+  major: int,
+  minor: int,
+  point: int,
+};
+
+let semver_re = Re.compile(Re.Posix.re({|([0-9]+)\.([0-9]+)\.([0-9]+)|}));
+
+let required_direnv_version = {
+  major: 2,
+  minor: 21,
+  point: 3
+};
+
+let required_nix_version = {
+  major: 2,
+  minor: 10,
+  point: 0
+};
+
+// TODO: test this
+let compare_version_number = (a, b) => {
+  switch (a, b) {
+  | (a, b) when a.major == b.major && a.minor == b.minor && a.point == b.point => 0
+  | (a, b) when a.major < b.major => -1
+  | (a, b) when a.major == b.major && a.minor < b.minor => -1
+  | (a, b) when a.major == b.major && a.minor == b.minor && a.point < b.point => -1
+  | _ => 1
+  }
+}
+
+let%test_unit "compare_version_number" = [%test_eq: int](compare_version_number({major: 1, minor: 0, point: 0}, {major: 2, minor: 0, point: 0}), -1);
+
+let extract_version_number = (cmd) => {
+  let full_cmd = cmd ++ " --version";
+  switch (Core_unix.open_process_in(full_cmd) |> In_channel.input_line) {
+  | Some(stdout) => {
+      let substrings = Re.exec(semver_re, stdout);
+      let groups = Re.Group.all(substrings);
+      Ok({
+        major: groups[1] |> int_of_string,
+        minor: groups[2] |> int_of_string,
+        point: groups[3] |> int_of_string
+      })
+    }
+  | None => Error(Printf.sprintf("Failed executing '%s'\n", cmd))
+  }
+};
+
+let is_version_new_enough = (cur, needed) => {
+  switch (cur) {
+    | Ok(cur) => {
+        switch (compare_version_number(cur, needed)) {
+        | x when x < 0 => Ok(false)
+        | _ => Ok(true)
+        }
+      }
+    | Error(e) => Error(e)
+  }
+}
+
+let preflight_versions = () => {
+  let in_direnv = switch (Sys.getenv("direnv")) {
+    | Some(_) => true
+    | None => false
+  };
+
+  let is_nix_new_enough = is_version_new_enough(extract_version_number("nix"), required_nix_version);
+  let is_direnv_new_enough = is_version_new_enough(extract_version_number("direnv"), required_direnv_version);
+
+  switch (in_direnv, is_direnv_new_enough, is_nix_new_enough) {
+    | (false, _, _) => Error("Not in direnv!")
+    | (_, Ok(false), _) => Error("Direnv version is not new enough")
+    | (_, _, Ok(false)) => Error("Nix version is not new enough")
+    | (_, Error(e), _) => Error(e)
+    | (_, _, Error(e)) => Error(e)
+    | (true, Ok(true), Ok(true)) => Ok()
+  }
+};