MetaLua

MetaLua is a very interesting metaprogramming Lua dialect.

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-- the "-{...}" means that we're going to do compile-time
-- stuff (here, syntax extension) 
---------------------------------------------------------
-{ block:
   ------------------------------------------------------
   -- Register the additional keywords in mlp.lexer
   ------------------------------------------------------
   mlp.lexer:add{ "let", "in" }

   ------------------------------------------------------
   -- Extend the expression parser; code generation is
   -- delegated to the function let_in_builder() below.
   ------------------------------------------------------
   mlp.expr:add{ 
     "let", mlp.id, "=", mlp.expr, "in", mlp.expr, 
     builder = let_in_builder }

   ------------------------------------------------------
   -- This creates the code returned by the macro.
   -- Notice the holes-in-quote-in-splice.
   ------------------------------------------------------
   local function let_in_builder (x)
     local variable, value, expr = unpack (x)
     return +{
       function (-{variable}) 
         return -{expr} 
       end (-{value}) }
   end
} -- back to "normal" code
 
a, b, c = 1, 1, -2 
roots = let sqrt_delta = (b^2-4*a*c)^0.5 in 
        { (sqrt_delta-b)/(2*a), (-sqrt_delta-b)/(2*a) }