lib/meta.nit

   1 # This file is part of NIT ( http://www.nitlanguage.org ).
   2 #
   3 # Licensed under the Apache License, Version 2.0 (the "License");
   4 # you may not use this file except in compliance with the License.
   5 # You may obtain a copy of the License at
   6 #
   7 #     http://www.apache.org/licenses/LICENSE-2.0
   8 #
   9 # Unless required by applicable law or agreed to in writing, software
  10 # distributed under the License is distributed on an "AS IS" BASIS,
  11 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  12 # See the License for the specific language governing permissions and
  13 # limitations under the License.
  14
  15 # Simple user-defined meta-level to manipulate types of instances as object.
  16 #
  17 # Unfortunately, since the meta-objects are user-defined they are provided without
  18 # any pre-defined information or behavior.
  19 # For the same reasons, the Nit OO mechanisms do not rely on this user-defined meta-level.
  20 #
  21 # However `meta` permits the definition of user-defined meta-classes at any level
  22 # of meta, even with user-defined meta-loops.
  23 #
  24 # ## Meta-classes
  25 #
  26 # Meta-classes can be defined easily in 3 steps:
  27 #
  28 # * define a root for the class hierarchy (eg `XObject`)
  29 # * define a meta-class (eq `XClass[E: XObject] super Class[E]`)
  30 # * redefine `CLASS` and `class_factory` in the root
  31 #
  32 # ~~~nitish because broke with nitc, see Limitation bellow
  33 # class XObject
  34 #    redef CLASS: XClass[SELF]
  35 #    redef class_factory(name) do return new XClass[SELF](name)
  36 # end
  37 # class XClass[E: XObject] super Class[E] end
  38 #
  39 # var x1 = new XObject
  40 # var x2 = new XObject
  41 # assert x1.get_class == x2.get_class
  42 # assert x1.get_class isa XClass[XObject]
  43 # assert x1.get_class.get_class isa Class[XClass[XObject]]
  44 # ~~~
  45 #
  46 # ## Limitation
  47 #
  48 # Currently works only with the interpreter `nit` and the compiler with `--erasure` (without `--rta`).
  49 #
  50 # `--rta` will try to detect all the runtime types, and will infinitely discover `Class[Class[Class[....]]]`.
  51 # Unfortunately, `--separate` and `--global` require `--rta`.
  52 #
  53 # Moreover, the interpreter and `--erasure` have a different behavior with generics since
  54 # with `--erasure` a single meta-instance is shared for all type variations of a same generic class.
  55 #
  56 # Class names are used as a primary key to identify classes.
  57 # But name conflicts are not managed and will make the program crashes at runtime (on some cast error)
  58 module meta
  59
  60 redef class Object
  61         # The meta-object representing the dynamic type of `self`.
  62         #
  63         # Specific meta-object can be used in subclasses
  64         # by redefining `CLASS` and `class_factory`.
  65         fun get_class: CLASS do
  66                 var class_pool = once new HashMap[String, Class[Object]]
  67                 var name = class_name
  68                 var res = class_pool.get_or_null(name)
  69                 if res != null then return res.as(CLASS)
  70                 res = class_factory(name)
  71                 assert res.name == name
  72                 class_pool[name] = res
  73                 return res
  74         end
  75
  76         # The type of the class of self.
  77         # To be redefined in case of specific meta-class.
  78         type CLASS: Class[SELF]
  79
  80         # Implementation used by `get_class` to create the specific class.
  81         #
  82         # To be redefined to use specific meta-classes.
  83         #
  84         # Note: Do not forget to update the virtual type `CLASS`.
  85         #
  86         # REQUIRE: `result.name` == `name`
  87         protected fun class_factory(name: String): CLASS
  88         do
  89                 return new Class[SELF](name)
  90         end
  91 end
  92
  93 # This meta-class is the root the meta-class hierarchy
  94 class Class[E: Object]
  95         # The name of the class
  96         var name: String
  97
  98         redef fun to_s do return name
  99 end