Allow the enabled_if field in library stanzas

CHANGES.md

@@ -65,6 +65,12 @@ unreleased
 - Make `dune subst` add a `(version ...)` field to the `dune-project`
   file (#2148, @diml)
 
+- Add the `%{os_type}` variable, which is a short-hand for
+  `%{ocaml-config:os_type}` (#1764, @diml)
+
+- Allow `enabled_if` fields in `library` stanzas, restricted to the
+  `%{os_type}` variable (#1764, @diml)
+
 1.9.3 (06/05/2019)
 ------------------
 

doc/dune-files.rst

@@ -209,6 +209,14 @@ to use the :ref:`include_subdirs` stanza.
   we were waiting for proper support for virtual libraries. Do not use
   in new code, it will be deleted in dune 2.0
 
+- ``(enabled_if <blang expression>)`` allows to conditionally disable
+  a library. A disabled library cannot be built and will not be
+  installed. The condition is specified using the blang_, and the
+  field allows for the ``%{os_type}`` variable, which is expanded to
+  the type of OS being targeted by the current build. Its value is
+  the same as the value of the ``os_type`` parameter in the output of
+  ``ocamlc -config``
+
 Note that when binding C libraries, dune doesn't provide special support for
 tools such as ``pkg-config``, however it integrates easily with configurator_ by
 using ``(c_flags (:include ...))`` and ``(c_library_flags (:include ...))``.
@@ -1087,6 +1095,8 @@ Dune supports the following variables:
 - ``profile`` the profile selected via ``--profile``
 - ``context_name`` the name of the context (``default`` or defined in the
   workspace file)
+- ``os_type`` is the type of the OS the build is targetting. This is
+  the same as ``ocaml-config:os_type``
 
 In addition, ``(action ...)`` fields support the following special variables:
 

src/blang.ml

@@ -102,3 +102,17 @@ let decode =
   and+ decode = decode
   in
   decode
+
+let rec fold_vars t ~init ~f =
+  match t with
+  | Const _ -> init
+  | Expr sw -> String_with_vars.fold_vars sw ~init ~f
+  | And l | Or l -> fold_vars_list l ~init ~f
+  | Compare (_, x, y) ->
+    String_with_vars.fold_vars y ~f
+      ~init:(String_with_vars.fold_vars x ~f ~init)
+
+and fold_vars_list ts ~init ~f =
+  match ts with
+  | [] -> init
+  | t :: ts -> fold_vars_list ts ~f ~init:(fold_vars t ~init ~f)

src/blang.mli

@@ -19,6 +19,8 @@ type t =
 
 val true_ : t
 
+val fold_vars : t -> init:'a -> f:(String_with_vars.Var.t -> 'a -> 'a) -> 'a
+
 val eval
   :  t
   -> dir:Path.t

src/context.ml

@@ -702,4 +702,5 @@ let lib_config t =
     has_native = has_native t
   ; ext_obj = t.ext_obj
   ; ext_lib = t.ext_lib
+  ; os_type = t.os_type
   }

src/dune_file.ml

@@ -361,10 +361,13 @@ module Preprocess = struct
               ]))
 end
 
-
-let enabled_if =
-  field "enabled_if" ~default:Blang.true_
-    (Syntax.since Stanza.syntax (1, 4) >>> Blang.decode)
+let enabled_if ~since =
+  let decode =
+    match since with
+    | None -> Blang.decode
+    | Some since -> Syntax.since Stanza.syntax since >>> Blang.decode
+  in
+  field "enabled_if" ~default:Blang.true_ decode
 
 module Per_module = struct
   include Per_item.Make(Module.Name)
@@ -896,6 +899,7 @@ module Library = struct
     ; private_modules          : Ordered_set_lang.t option
     ; stdlib                   : Stdlib.t option
     ; special_builtin_support  : Special_builtin_support.t option
+    ; enabled_if               : Blang.t
     }
 
   let decode =
@@ -956,7 +960,7 @@ module Library = struct
          field_o "special_builtin_support"
            (Syntax.since Stanza.syntax (1, 10) >>>
             Special_builtin_support.decode)
-       in
+       and+ enabled_if = enabled_if ~since:(Some (1, 10)) in
        let wrapped = Wrapped.make ~wrapped ~implements in
        let name =
          let open Syntax.Version.Infix in
@@ -1021,6 +1025,14 @@ module Library = struct
                  "A library cannot use (self_build_stubs_archive ...) \
                   and (%s ...) simultaneously." name
            in
+           Blang.fold_vars enabled_if ~init:() ~f:(fun var () ->
+             match String_with_vars.Var.name var,
+                   String_with_vars.Var.payload var with
+             | "os_type", None -> ()
+             | _ ->
+               Errors.fail (String_with_vars.Var.loc var)
+                 "Only the 'os_type' variable is allowed in the 'enabled_if' \
+                  field of libraries.");
            { name
            ; public
            ; synopsis
@@ -1050,6 +1062,7 @@ module Library = struct
            ; private_modules
            ; stdlib
            ; special_builtin_support
+           ; enabled_if
            })
 
   let has_stubs t =
@@ -1659,7 +1672,7 @@ module Rule = struct
           | false, Some mode -> Ok mode
           | true, None -> Ok Fallback
           | false, None -> Ok Standard)
-    and+ enabled_if = enabled_if
+    and+ enabled_if = enabled_if ~since:(Some (1, 4))
     in
     { targets
     ; deps
@@ -1732,7 +1745,7 @@ module Rule = struct
             record
               (let+ modules = field "modules" (list string)
                and+ mode = Mode.field
-               and+ enabled_if = enabled_if
+               and+ enabled_if = enabled_if ~since:(Some (1, 4))
                in
                { modules; mode; enabled_if }))
           ~else_:(
@@ -1818,7 +1831,7 @@ module Menhir = struct
        and+ mode = Rule.Mode.field
        and+ infer = field_o_b "infer" ~check:(Syntax.since syntax (2, 0))
        and+ menhir_syntax = Syntax.get_exn syntax
-       and+ enabled_if = enabled_if
+       and+ enabled_if = enabled_if ~since:(Some (1, 4))
        and+ loc = loc
        in
        let infer =
@@ -1911,7 +1924,7 @@ module Coq = struct
        and+ modules = modules_field "modules"
        and+ libraries =
          field "libraries" (list (located Lib_name.decode)) ~default:[]
-       and+ enabled_if = enabled_if
+       and+ enabled_if = enabled_if ~since:None
        in
        let name =
          let (loc, res) = name in
@@ -2016,7 +2029,7 @@ module Tests = struct
                      ~default:Executables.Link_mode.Set.default
        and+ deps =
          field "deps" (Bindings.decode Dep_conf.decode) ~default:Bindings.empty
-       and+ enabled_if = enabled_if
+       and+ enabled_if = enabled_if ~since:(Some (1, 4))
        and+ action =
          field_o
            "action"

src/dune_file.mli

@@ -235,6 +235,7 @@ module Library : sig
     ; private_modules          : Ordered_set_lang.t option
     ; stdlib                   : Stdlib.t option
     ; special_builtin_support  : Special_builtin_support.t option
+    ; enabled_if               : Blang.t
     }
 
   val has_stubs : t -> bool

src/lib.ml

@@ -891,12 +891,16 @@ let rec instantiate db name (info : Lib_info.t) ~stack ~hidden =
   let res =
     let hidden =
       match hidden with
-      | None ->
-        Option.some_if
-          (info.optional &&
-           not (Result.is_ok t.requires && Result.is_ok t.ppx_runtime_deps))
-          "optional with unavailable dependencies"
       | Some _ -> hidden
+      | None ->
+        match info.enabled with
+        | Normal -> None
+        | Optional ->
+          Option.some_if
+            (not (Result.is_ok t.requires && Result.is_ok t.ppx_runtime_deps))
+            "optional with unavailable dependencies"
+        | Disabled_because_of_enabled_if ->
+          Some "unsatisfied 'enabled_if'"
     in
     match hidden with
     | None -> St_found t
@@ -1265,7 +1269,9 @@ module Compile = struct
       make_lib_deps_info
         ~user_written_deps:(Lib_info.user_written_deps t.info)
         ~pps:t.info.pps
-        ~kind:(Lib_deps_info.Kind.of_optional t.info.optional)
+        ~kind:(match t.info.enabled with
+          | Normal -> Required
+          | _ -> Optional)
     in
     let requires_link = lazy (
       t.requires >>= closure_with_overlap_checks

src/lib_config.ml

@@ -4,4 +4,5 @@ type t =
   { has_native : bool
   ; ext_lib : string
   ; ext_obj : string
+  ; os_type : string
   }

src/lib_info.ml

@@ -47,6 +47,13 @@ module Source = struct
     | External of 'a
 end
 
+module Enabled_status = struct
+  type t =
+    | Normal
+    | Optional
+    | Disabled_because_of_enabled_if
+end
+
 type t =
   { loc              : Loc.t
   ; name             : Lib_name.t
@@ -66,7 +73,7 @@ type t =
   ; requires         : Deps.t
   ; ppx_runtime_deps : (Loc.t * Lib_name.t) list
   ; pps              : (Loc.t * Lib_name.t) list
-  ; optional         : bool
+  ; enabled          : Enabled_status.t
   ; virtual_deps     : (Loc.t * Lib_name.t) list
   ; dune_version     : Syntax.Version.t option
   ; sub_systems      : Sub_system_info.t Sub_system_name.Map.t
@@ -86,7 +93,7 @@ let user_written_deps t =
     ~f:(fun acc s -> Dune_file.Lib_dep.Direct s :: acc)
 
 let of_library_stanza ~dir
-      ~lib_config:{ Lib_config.has_native; ext_lib; ext_obj}
+      ~lib_config:{ Lib_config.has_native; ext_lib; ext_obj; os_type}
       (conf : Dune_file.Library.t) =
   let (_loc, lib_name) = conf.name in
   let obj_dir =
@@ -154,6 +161,21 @@ let of_library_stanza ~dir
   let main_module_name = Dune_file.Library.main_module_name conf in
   let name = Dune_file.Library.best_name conf in
   let modes = Dune_file.Mode_conf.Set.eval ~has_native conf.modes in
+  let enabled =
+    let enabled_if_result =
+      Blang.eval conf.enabled_if ~dir ~f:(fun v _ver ->
+        match String_with_vars.Var.name v,
+              String_with_vars.Var.payload v with
+        | "os_type", None -> Some [String os_type]
+        | _ -> None)
+    in
+    if not enabled_if_result then
+      Enabled_status.Disabled_because_of_enabled_if
+    else if conf.optional then
+      Optional
+    else
+      Normal
+  in
   { loc = conf.buildable.loc
   ; name
   ; kind     = conf.kind
@@ -164,7 +186,7 @@ let of_library_stanza ~dir
   ; synopsis = conf.synopsis
   ; archives
   ; plugins
-  ; optional = conf.optional
+  ; enabled
   ; foreign_objects
   ; foreign_archives
   ; jsoo_runtime
@@ -219,7 +241,7 @@ let of_dune_lib dp =
   ; jsoo_runtime = Lib.jsoo_runtime dp
   ; jsoo_archive = None
   ; pps = []
-  ; optional = false
+  ; enabled = Normal
   ; virtual_deps = []
   ; dune_version = None
   ; sub_systems = Lib.sub_systems dp

src/lib_info.mli

@@ -28,6 +28,13 @@ module Source : sig
     | External of 'a
 end
 
+module Enabled_status : sig
+  type t =
+    | Normal
+    | Optional
+    | Disabled_because_of_enabled_if
+end
+
 type t = private
   { loc              : Loc.t
   ; name             : Lib_name.t
@@ -47,7 +54,7 @@ type t = private
   ; requires         : Deps.t
   ; ppx_runtime_deps : (Loc.t * Lib_name.t) list
   ; pps              : (Loc.t * Lib_name.t) list
-  ; optional         : bool
+  ; enabled          : Enabled_status.t
   ; virtual_deps     : (Loc.t * Lib_name.t) list
   ; dune_version     : Syntax.Version.t option
   ; sub_systems      : Sub_system_info.t Sub_system_name.Map.t

src/pform.ml

@@ -208,6 +208,8 @@ module Map = struct
       ; "workspace_root" , values [Value.Dir context.build_dir]
       ; "context_name"   , string (Context.name context)
       ; "ROOT"           , renamed_in ~version:(1, 0) ~new_name:"workspace_root"
+      ; "os_type"        , since ~version:(1, 10)
+                             (Var.Values [String context.os_type])
       ]
     in
     { vars =

src/scope.ml

@@ -130,13 +130,14 @@ module DB = struct
       ~f:(fun _name project libs ->
         let project = Option.value_exn project in
         let libs = Option.value libs ~default:[] in
-        let db = Lib.DB.create_from_library_stanzas libs
-                   ~parent:public_libs ~lib_config in
+        let db = Lib.DB.create_from_library_stanzas libs ~parent:public_libs
+                   ~lib_config in
         let root =
           Path.append_source build_context_dir (Dune_project.root project) in
         Some { project; db; root })
 
-  let create ~projects ~context ~installed_libs ~lib_config internal_libs =
+  let create ~projects ~context ~installed_libs ~lib_config
+        internal_libs =
     let t = Fdecl.create () in
     let public_libs = public_libs t ~installed_libs internal_libs in
     let by_name =

src/string_with_vars.ml

@@ -253,6 +253,15 @@ let known_prefix =
     | Var v :: _ -> Partial (String.concat ~sep:"" (List.rev acc), v)
   in
   fun t -> go t.template.parts []
+let fold_vars =
+  let rec loop parts acc f =
+    match parts with
+    | [] -> acc
+    | Text _ :: parts -> loop parts acc f
+    | Var v :: parts -> loop parts (f v acc) f
+  in
+  fun t ~init ~f ->
+    loop t.template.parts init f
 
 type 'a expander = Var.t -> Syntax.Version.t -> 'a
 

src/string_with_vars.mli

@@ -93,6 +93,7 @@ val known_prefix : t -> known_prefix
 val is_suffix : t -> suffix:string -> yes_no_unknown
 
 val is_prefix : t -> prefix:string -> yes_no_unknown
+val fold_vars : t -> init:'a -> f:(Var.t -> 'a -> 'a) -> 'a
 
 type 'a expander = Var.t -> Syntax.Version.t -> 'a
 

test/blackbox-tests/test-cases/enabled_if/dune

@@ -21,3 +21,22 @@
           (echo "Building file b")
           (with-stdout-to b (progn))))
  (enabled_if true))
+
+(library
+ (name foo)
+ (modules)
+ (enabled_if false))
+
+(library
+ (name bar)
+ (modules)
+ (libraries foo))
+
+(library
+ (name baz)
+ (modules)
+ (libraries bar))
+
+(rule (with-stdout-to main.ml (echo "")))
+
+(executable (name main) (libraries baz))

test/blackbox-tests/test-cases/enabled_if/dune-project

@@ -1 +1 @@
-(lang dune 1.4)
+(lang dune 1.10)