--- /dev/null
+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Simple user-defined meta-level to manipulate types of instances as object.
+#
+# Unfortunately, since the meta-objects are user-defined they are provided without
+# any pre-defined information or behavior.
+# For the same reasons, the Nit OO mechanisms do not rely on this user-defined meta-level.
+#
+# However `meta` permits the definition of user-defined meta-classes at any level
+# of meta, even with user-defined meta-loops.
+#
+# ## Meta-classes
+#
+# Meta-classes can be defined easily in 3 steps:
+#
+# * define a root for the class hierarchy (eg `XObject`)
+# * define a meta-class (eq `XClass[E: XObject] super Class[E]`)
+# * redefine `CLASS` and `class_factory` in the root
+#
+# ~~~nitish because broke with nitc, see Limitation bellow
+# class XObject
+# redef CLASS: XClass[SELF]
+# redef class_factory(name) do return new XClass[SELF](name)
+# end
+# class XClass[E: XObject] super Class[E] end
+#
+# var x1 = new XObject
+# var x2 = new XObject
+# assert x1.get_class == x2.get_class
+# assert x1.get_class isa XClass[XObject]
+# assert x1.get_class.get_class isa Class[XClass[XObject]]
+# ~~~
+#
+# ## Limitation
+#
+# Currently works only with the interpreter `nit` and the compiler with `--erasure` (without `--rta`).
+#
+# `--rta` will try to detect all the runtime types, and will infinitely discover `Class[Class[Class[....]]]`.
+# Unfortunately, `--separate` and `--global` require `--rta`.
+#
+# Moreover, the interpreter and `--erasure` have a different behavior with generics since
+# with `--erasure` a single meta-instance is shared for all type variations of a same generic class.
+#
+# Class names are used as a primary key to identify classes.
+# But name conflicts are not managed and will make the program crashes at runtime (on some cast error)
+module meta
+
+redef class Object
+ # The meta-object representing the dynamic type of `self`.
+ #
+ # Specific meta-object can be used in subclasses
+ # by redefining `CLASS` and `class_factory`.
+ fun get_class: CLASS do
+ var class_pool = once new HashMap[String, Class[Object]]
+ var name = class_name
+ var res = class_pool.get_or_null(name)
+ if res != null then return res.as(CLASS)
+ res = class_factory(name)
+ assert res.name == name
+ class_pool[name] = res
+ return res
+ end
+
+ # The type of the class of self.
+ # To be redefined in case of specific meta-class.
+ type CLASS: Class[SELF]
+
+ # Implementation used by `get_class` to create the specific class.
+ #
+ # To be redefined to use specific meta-classes.
+ #
+ # Note: Do not forget to update the virtual type `CLASS`.
+ #
+ # REQUIRE: `result.name` == `name`
+ protected fun class_factory(name: String): CLASS
+ do
+ return new Class[SELF](name)
+ end
+end
+
+# This meta-class is the root the meta-class hierarchy
+class Class[E: Object]
+ # The name of the class
+ var name: String
+
+ redef fun to_s do return name
+end
--- /dev/null
+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import meta
+
+class XObject
+ redef type CLASS: XClass[SELF]
+ redef fun class_factory(name) do return new XClass[SELF](name)
+end
+
+class XClass[E: XObject]
+ super Class[E]
+end
+
+class YObject
+ redef type CLASS: YClass[SELF]
+ redef fun class_factory(name) do return new YClass[SELF](name)
+end
+
+class YClass[E: YObject]
+ super Class[E]
+ super YObject # Yeah, a meta-loop!
+end
+
+var s = "hello"
+print s.class_name
+print s.get_class
+print s.get_class.get_class
+print s.get_class.get_class.get_class
+
+print ""
+
+var x = new XObject
+print x.class_name
+print x.get_class
+print x.get_class.get_class
+print x.get_class.get_class.get_class
+
+print ""
+
+var y = new YObject
+print y.class_name
+print y.get_class
+print y.get_class.get_class
+print y.get_class.get_class.get_class
nitlanguage / nit.git / commitdiff