Make CLI-parsing types more readable

Cf. comments in #11.

Also, the change in `examples.ml` allows people exploring the example to
easily just get the interesting type from Merlin:

```ocaml
(string Genspio.EDSL.t ->
 bool Genspio.EDSL.t ->
 string Genspio.EDSL.t -> string Genspio.EDSL.t -> unit Genspio.EDSL.t,
 unit Genspio.EDSL.t)
Genspio.EDSL.cli_options
```

src/lib/EDSL.mli

@@ -97,9 +97,9 @@ val feed : string:string t -> unit t -> unit t
 val ( >> ) : string t -> unit t -> unit t
 val loop_while : bool t -> body:unit t -> unit t
 
-type 'a cli_option = 'a Language.cli_option
-type 'a option_spec = 'a Language.option_spec
-type ('a, 'b) cli_options = ('a, 'b) Language.cli_options
+type 'argument_type cli_option = 'argument_type Language.cli_option
+type 'argument_type option_spec = 'argument_type Language.option_spec
+type ('parse_function, 'return_type) cli_options = ('parse_function, 'return_type) Language.cli_options
 module Option_list : sig
   val string :
     ?default: string t ->
@@ -110,13 +110,14 @@ module Option_list : sig
     doc:string -> char ->
     bool t option_spec
   val ( & ) :
-    'a option_spec ->
-    ('b, 'c) cli_options -> ('a -> 'b, 'c) cli_options
-  val usage : string -> ('a, 'a) cli_options
+    'argument_type option_spec ->
+    ('parse_function, 'return_type) cli_options ->
+    ('argument_type -> 'parse_function, 'return_type) cli_options
+  val usage : string -> ('last_return_type, 'last_return_type) cli_options
 end
 
 val parse_command_line :
-  ('a, unit t) cli_options -> 'a -> unit t
+  ('parse_function, unit t) cli_options -> 'parse_function -> unit t
 
 val string_concat: string t list -> string t
 

src/test/examples.ml

@@ -133,7 +133,7 @@ let downloader () =
     ]
   end in
   let no_value = sprintf "none_%x" (Random.int 100_000) |> string in
-  parse_command_line
+  let cli_spec =
     Option_list.(
       string
         ~doc:"The URL to the stuff" 'u'
@@ -146,7 +146,8 @@ let downloader () =
         ~doc:"Use <dir> as temp-dir"
         ~default:(Genspio.EDSL.string "/tmp/genspio-downloader-tmpdir")
       & usage "$0 -u URL [-c]"
-    )
+    ) in
+  parse_command_line cli_spec
     begin fun url all_in_tmp filename_ov tmp_dir ->
       let current_name = tmp_file ~tmp_dir "current-name" in
       let set_output_of_download () =