Add fowl's OOP macro

src/SUMMARY.md

@@ -12,3 +12,4 @@
 * [Exceptions](exceptions.md)
 * [Functions]()
 * [Varargs](varargs.md)
+* [OOP Macro](oop_macro.md)

src/oop_macro.md

@@ -0,0 +1,219 @@
+<!--- Thanks to fowl for creating this page -->
+
+# OOP Macro
+This is the code that we currently must write to use OOP in nimrod:
+```nimrod
+type Animal =
+  ref object {.inheritable.}
+    name: string
+method vocalize(self: Animal): string = "..."
+
+type Dog =
+  ref object of Animal
+    lastRabiesDate: int
+method vocalize(self: Dog): string = "woof"
+
+type Cat = ref object of Animal
+method vocalize(self: Cat): string = "meow"
+```
+
+All these typedefs and `self: T` parameters are repetitive, so it'd be good to write a macro to mask them. Something like this would be best:
+
+```nimrod
+class(Animal of TObject):
+  var name: string
+  method vocalize: string = "..."
+
+class(Dog of Animal):
+  var lastRabiesDate: int
+  method vocalize: string = "woof"
+
+class(Cat of Animal):
+  method vocalize: string = "woof"
+
+# a short test (used later)
+proc run_test() =
+  var pets: seq[Animal] = @[]
+  pets.add Cat(name: "meowth")
+  pets.add Dog(name: "ruffles")
+  assert(@["meow", "woof"] == @[pets[0].vocalize,
+                                pets[1].vocalize])
+```
+
+```nimrod
+import macros
+
+macro class*(): stmt {.immediate.} =
+  # The macro is immediate so that it doesn't
+  # resolve identifiers passed to it
+
+  let cs = callsite()
+  var
+    class_name, in_stmts: PNimrodNode
+    superclass: PNimrodNode
+
+  # Manual parsing of arguments. Here we expect
+  # a class name and attached stmts list
+  if cs[1].kind == nnkInfix and $cs[1][0] == "of":
+    # the expression is "class_name" of "superclass"
+    # echo cs[1].treerepr
+    # -------------------
+    # Infix
+    #   Ident !"of"
+    #   Ident !"Animal"
+    #   Ident !"TObject"
+
+    class_name = cs[1][1]
+    superclass = cs[1][2]
+  else:
+    class_name = cs[1]
+
+  in_stmts = cs[2]
+
+  if in_stmts.kind != nnkStmtList:
+    quit "Malformed arguments for class() macro: expected nnkStmtList, got " &
+      lisprepr(in_stmts)
+  assert class_name.kind == nnkIdent
+
+  # echo "class_name: ", treerepr(class_name)
+  # -----------------------------------------
+  # class_name: Ident !"Animal"
+
+  # echo "stmts: ", treerepr(in_stmts)
+  # ----------------------------------
+  # stmts:
+  # StmtList
+  #   VarSection
+  #     IdentDefs
+  #       Ident !"name"
+  #       Ident !"string"
+  #       Empty
+  #     IdentDefs
+  #       Ident !"age"
+  #       Ident !"int"
+  #       Empty
+  #   MethodDef
+  #     Ident !"vocalize"
+  #     Empty
+  #     Empty
+  #     FormalParams
+  #       Ident !"string"
+  #     Empty
+  #     Empty
+  #     StmtList
+  #       StrLit ...
+  #   MethodDef
+  #     Ident !"age_human_yrs"
+  #     Empty
+  #     Empty
+  #     FormalParams
+  #       Ident !"int"
+  #     Empty
+  #     Empty
+  #     StmtList
+  #       DotExpr
+  #         Ident !"self"
+  #         Ident !"age"
+
+  result = newStmtList()
+  result.add newEmptyNode()
+  # create a new stmtList for the result,
+  # the first slot is a placeholder
+  # the type definition we generate
+
+  var rec_list = newNimNode(nnkRecList)
+  # var declarations will be turned into object fields
+
+  # Iterate over the statements, adding `self: T`
+  # to the parameters of functions
+  for node in children(in_stmts):
+    case node.kind
+    of nnkMethodDef, nnkProcDef:
+      # inject self:T into the arguments
+      let p = node.params.copyNimTree
+      p.insert(1, newIdentDefs(ident"self", class_name))
+      node.params = p
+      result.add node
+
+    of nnkVarSection:
+      # variables get turned into fields on the type,
+      # isn't that neat?
+      # here we just collect them though
+      for N in children(node):
+        assert n.kind == nnkIdentDefs, "Invalid node " & lispRepr(N)
+        rec_list.add N
+
+    else:
+      result.add node
+
+  # The following prints out the AST structure:
+  #
+  # import macros
+  # dumptree:
+  #   type X = ref object of Y
+  #     z: int
+  # --------------------------
+  # TypeSection
+  #   TypeDef
+  #     Ident !"X"
+  #     Empty
+  #     RefTy
+  #       ObjectTy
+  #         Empty
+  #         OfInherit
+  #           Ident !"Y"
+  #         RecList
+  #           IdentDefs
+  #             Ident !"z"
+  #             Ident !"int"
+  #             Empty
+
+  var obj_ty = newNimNode(nnkObjectTy).add(
+    newEmptyNode(),
+    (if superclass.isNil: newEmptyNode() else: newNimNode(nnkOfInherit).add(superclass)),
+    rec_list
+  )
+  result[0] = newNimNode(nnkTypeSection).add(
+    newNimNode(nnkTypeDef).add(
+      class_name,
+      newEmptyNode(),
+      newNimNode(nnkRefTy).add(obj_ty)
+  ))
+
+  # Lets inspect the human-readable version of the output
+  # echo repr(result)
+  # Output:
+  #  type
+  #    Animal = ref object of TObject
+  #      name: string
+  #      age: int
+  #
+  #  method vocalize(self: Animal): string =
+  #    "..."
+  #
+  #  method age_human_yrs(self: Animal): int =
+  #    self.age
+
+  # more could be done here, it could be made an
+  # option to use ref types, export type names, etc
+
+
+class(Animal of TObject):
+  var
+    name: string
+    age: int
+  method vocalize: string = "..."
+  method age_human_yrs: int = self.age
+    # self is injected
+
+class(Dog of Animal):
+  method vocalize: string = "woof"
+  method age_human_yrs: int = self.age * 7
+
+class(Cat of Animal) do:
+  method vocalize: string = "meow"
+
+
+run_test()
+echo "Tests passed!"
+```