more explicit error

src/grew_ast.ml

@@ -89,12 +89,23 @@ module Ast = struct
     | Equal_param of string (* $ident *)
     | Absent
 
+  let feature_kind_to_string = function
+    | Equality fv_list -> sprintf " = %s" (String.concat "|" fv_list)
+    | Disequality [] -> ""
+    | Disequality fv_list -> sprintf " <> %s" (String.concat "|" fv_list)
+    | Equal_param param -> sprintf  " = $%s" param
+    | Absent -> " <> *"
+
   type u_feature = {
     name: feature_name;
     kind: feature_kind;
   }
+  let u_feature_to_string uf =
+    sprintf "%s%s" uf.name (feature_kind_to_string uf.kind)
+
   type feature = u_feature * Loc.t
 
+
   let default_fs ?loc lab =
     match loc with
     | None -> [({name="label"; kind=Equality [lab]}, Loc.empty)]

src/grew_ast.mli

@@ -61,6 +61,8 @@ module Ast : sig
     name: feature_name;
     kind: feature_kind;
   }
+  val u_feature_to_string: u_feature -> string
+
   type feature = u_feature * Loc.t
 
   val default_fs: ?loc:Loc.t -> string -> feature list

src/grew_fs.ml

@@ -51,7 +51,7 @@ module G_feature = struct
   let build ?domain = function
     | ({Ast.kind=Ast.Equality [atom]; name=name},loc) ->
       (name, Feature_value.build_value ~loc ?domain name atom)
-    | _ -> Error.build "Illegal feature declaration in Graph (must be '=' and atomic)"
+    | (uf,loc) -> Error.build ~loc "in graph nodes, features must follow the shape \"name = value\" (error on feature: \"%s\")" (Ast.u_feature_to_string uf)
 
   let to_string (feat_name, feat_val) = sprintf "%s=%s" feat_name (string_of_value feat_val)