add shell.nix and fix some deps

shell.nix

@@ -0,0 +1,31 @@
+{ pkgs ? import <nixpkgs> { } }:
+
+let
+  ocamlPackages = pkgs.ocaml-ng.ocamlPackages_5_3;
+in
+pkgs.mkShell {
+  name = "frost";
+  dontDetectOcamlConflicts = false;
+  nativeBuildInputs = with ocamlPackages; [
+    dune_3
+    findlib
+    merlin
+    ocaml
+    ocamlformat
+    odoc
+  ];
+  buildInputs = with ocamlPackages; [
+    bos
+    cmdliner
+    directories
+    fmt
+    fpath
+    ptime
+    prelude
+    scfg
+  ];
+  shellHook = ''
+    export PATH=$PATH:${pkgs.lib.makeBinPath [
+    ]}
+  '';
+}

src/dune

@@ -2,8 +2,11 @@
  (public_name sun)
  (libraries
   astring
+  bos
   cmdliner
   directories
+  fmt
+  fpath
   scfg
   prelude
   ptime

src/sun.ml

@@ -15,7 +15,7 @@ let choose_output = function
         let module Project_dirs = Directories.Project_dirs (App_id) in
         match Project_dirs.config_dir with
         | None -> Fmt.failwith "can't compute configuration directory path"
-        | Some config_dir -> Fpath.v config_dir
+        | Some config_dir -> config_dir
       in
       let file_name = Fpath.(config_dir / "config.scfg") in
       (* if the config file doesn't exist, then we use the empty configuration *)
@@ -38,11 +38,7 @@ let choose_output = function
         (* If we can't compute `picture_dir`, we use the current working directory. *)
         begin
           match User_dirs.picture_dir with
-          | Some dir -> begin
-            match Fpath.of_string dir with
-            | Error (`Msg e) -> Fmt.failwith "%s" e
-            | Ok dir -> dir
-          end
+          | Some dir -> dir
           | None -> (
             match Bos.OS.Dir.current () with
             | Error (`Msg e) -> Fmt.failwith "%s" e