describe: add contexts subcommand

Signed-off-by: Javier Chávarri <[email protected]>

bin/describe/describe.ml

@@ -19,6 +19,7 @@ let subcommands =
   ; Aliases_targets.Aliases_cmd.command
   ; Package_entries.command
   ; Describe_pkg.command
+  ; Describe_contexts.command
   ]
 ;;
 

bin/describe/describe_contexts.ml

@@ -0,0 +1,23 @@
+open Import
+
+let term =
+  let+ builder = Common.Builder.term in
+  let common, config = Common.init builder in
+  Scheduler.go ~common ~config
+  @@ fun () ->
+  let open Fiber.O in
+  let* setup = Import.Main.setup () in
+  let+ setup = Memo.run setup in
+  let ctxts =
+    List.map
+      ~f:(fun (name, _) -> Context_name.to_string name)
+      (Context_name.Map.to_list setup.scontexts)
+  in
+  List.iter ctxts ~f:print_endline
+;;
+
+let command =
+  let doc = "prints information about the available Dune contexts." in
+  let info = Cmd.info ~doc "contexts" in
+  Cmd.v info term
+;;

bin/describe/describe_contexts.mli

@@ -0,0 +1,4 @@
+open Import
+
+(** Dune command to print out the defined Dune contexts.*)
+val command : unit Cmd.t

test/blackbox-tests/test-cases/describe/describe-contexts.t

@@ -0,0 +1,19 @@
+Showcase behavior of the `dune describe contexts` subcommand
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.14)
+  > EOF
+
+  $ cat > dune-workspace << EOF
+  > (lang dune 3.14)
+  > 
+  > (context default)
+  > 
+  > (context
+  >  (default
+  >   (name alt)))
+  > EOF
+
+  $ dune describe contexts
+  alt
+  default