hints/assignment_4_hint_3.mdwn

   1 Hints for Y1,Y2.
   2
   3 *       Begin by trying to define `X1` and `X2` directly [[assignment 4 hint 3 hint 1]], then once they're working, you can abstract back to a pair of combinators `Y1` and `Y2` that generate `X1` and `X2` for a given pair of arguments `T1` and `T2`.
   4
   5         You want:
   6
   7                 X1 <~~> T1 X1 X2 and
   8                 X2 <~~> T2 X1 X2
   9
  10 *       X1 is presumably going to have some form like:
  11
  12                 (\elem1 elem2 ... (T1
  13                                                         (do-something-with-the-elems-to-make-X1)
  14                                                         (do-something-with-the-elems-to-make-X2)
  15                                                   ))
  16                 some-elem1 some-elem2 ...
  17
  18         Right? What will the form of X2 be?
  19
  20 *       Can you see how to go on?
  21
  22 *       Call the displayed abstract above `(\elem1 elem2 ... (...))` "elem0". What's the relation between `X1`, `elem0`, `elem1`, `elem2`...?
  23
  24 *       Go on, now you can do it.
  25
  26