Simple AST walker with multi-child AST

gistfile1.vim

" VimL Example of Walking a multi-child AST
" Barry Arthur, 2012 08 01

" The same simple AST walker
func! Walk(ast, visitor)
  return type(a:ast) == type([]) ? call(a:visitor[a:ast[0]], [a:ast[1]], a:visitor) : a:ast
endfunc

" rainbows and unicorns
function! RnU()
  let rnu = {}
  func rnu.print(args) dict
    let lst = []
    for arg in a:args
      call add(lst, Walk(arg, self))
      unlet arg
    endfor
    return join(lst, ' ')
  endfunc
  func rnu.upper(args) dict
    let lst = []
    for arg in a:args
      call add(lst, toupper(Walk(arg, self)))
      unlet arg
    endfor
    return join(lst, ' ')
  endfunc
  return rnu
endfunction

let ast = ['print', ['Unicorns', 'now', 'served', 'with', ['upper', ['lots', "of", 'extra', 'cheese']], 'and', 'sauce!']]

echo Walk(ast, RnU())