lib/metalua/extension/anaphoric.mlua

-------------------------------------------------------------------------------
-- Copyright (c) 2006-2013 Fabien Fleutot and others.
--
-- All rights reserved.
--
-- This program and the accompanying materials are made available
-- under the terms of the Eclipse Public License v1.0 which
-- accompanies this distribution, and is available at
-- http://www.eclipse.org/legal/epl-v10.html
--
-- This program and the accompanying materials are also made available
-- under the terms of the MIT public license which accompanies this
-- distribution, and is available at http://www.lua.org/license.html
--
-- Contributors:
--     Fabien Fleutot - API and implementation
--
-------------------------------------------------------------------------------

--------------------------------------------------------------------------------
--
-- Anaphoric macros.
--
-- This extension turns 'it' into a special variable, that's bound to
-- an often used value:
--
-- * in an 'if' statement, 'it' is bound, in a block, to the condition
--   that triggered the block's execution:
--   > if 1234 then y=it end; assert (y == 1234)
--
-- * in a while loop, it's bound to the test:
--   > while file:read "*a" do table.insert (lines, it) end
--
-- 'it' is bound the the most closely surrounding structure. If you wanted to
-- use its content at a deeper position in the AST, you would have to save it
-- in a temporary variable. But what you should really do in such a case is
-- avoiding to use anaphoric macros: they're fine for one-liner, but they
-- reduce readability for bigger functions.
--------------------------------------------------------------------------------

-- TODO: 'and' operator could, and maybe should, be anaphoric as well
-- TODO: anaphoric functions would be cool for recursive functions, but
--       recursive calls are always in an 'if' statement, so the pronoun
--       used for functions must not be the same as for 'if'.

require 'freevars'

local function anaphoric_if(ast)
   local it_found = false
   for i=2, #ast do 
      if freevars.block(ast[i])['it'] then
         it_found = true
         break
      end
   end
   if it_found then
      local cond = ast[1]
      ast[1] = +{it}
      return +{stat: do local it = -{cond}; -{ast} end }
   end
end

local function anaphoric_while(ast)
   local it_found = false
   if freevars.block(ast[2])['it'] then
      local cond = ast[1]
      ast[1] = +{it}
      return +{stat: do local it = -{cond}; -{ast} end }
   end
end

mlp.stat:get'if'.transformers:add(anaphoric_if)
mlp.stat:get'while'.transformers:add(anaphoric_while)