# Store a function definition
#
# Example:
#
#      f(x,y)=x^y
#
# For this definition, p1 points to the following structure.
#
#     p1
#      |
#   ___v__    ______                        ______ 
#  |CONS  |->|CONS  |--------------------->|CONS  |
#  |______|  |______|                      |______|
#      |         |                             |
#   ___v__    ___v__    ______    ______    ___v__    ______    ______
#  |SETQ  |  |CONS  |->|CONS  |->|CONS  |  |CONS  |->|CONS  |->|CONS  |
#  |______|  |______|  |______|  |______|  |______|  |______|  |______|
#                |         |         |         |         |         |
#             ___v__    ___v__    ___v__    ___v__    ___v__    ___v__
#            |SYM f |  |SYM x |  |SYM y |  |POWER |  |SYM x |  |SYM y |
#            |______|  |______|  |______|  |______|  |______|  |______|
#
# the result (in f) is a FUNCTION node
# that contains both the body and the argument list.
#
# We have
#
#  caadr(p1) points to the function name i.e. f
#  cdadr(p1) points to the arguments i.e. the list (x y)
#  caddr(p1) points to the function body i.e. (power x y)



#define F p3 # F points to the function name
#define A p4 # A points to the argument list
#define B p5 # B points to the function body

define_user_function = ->
  p3 = caadr(p1); # p3 is F
  p4 = cdadr(p1); # p4 is A
  p5 = caddr(p1); # p5 is B

  if (!issymbol(p3)) # p3 is F
    stop("function name?")

  # evaluate function body (maybe)

  if (car(p5) == symbol(EVAL))  # p5 is B
    push(cadr(p5));  # p5 is B
    Eval()
    p5 = pop();  # p5 is B

  # note how, unless explicitly forced by an eval,
  # (handled by the if just above)
  # we don't eval/simplify
  # the body.
  # Why? because it's the easiest way
  # to solve scope problems i.e.
  #   x = 0
  #   f(x) = x + 1
  #   f(4) # would reply 1
  # which would need to otherwise
  # be solved by some scope device
  # somehow
  push_symbol(FUNCTION)
  push p5
  push p4
  list(3)
  p5 = pop()


  set_binding(p3, p5);  # p3 is F (function name)  # p4 is A  # p5 is B

  # return value is nil

  push_symbol(NIL)

Eval_function_reference = ->
  push p1