Weak Head Normal Form

    (WHNF) A lambda expression is in weak head
   normal form (WHNF) if it is a head normal form (HNF) or any
   lambda abstraction.  I.e. the top level is not a redex.

   The term was coined by Simon Peyton Jones to make explicit
   the difference between head normal form (HNF) and what
   graph reduction systems produce in practice.  A lambda
   abstraction with a reducible body, e.g.

   	\ x . ((\ y . y+x) 2)

   is in WHNF but not HNF.  To reduce this expression to HNF
   would require reduction of the lambda body:

   	(\ y . y+x) 2  -->  2+x

   Reduction to WHNF avoids the name capture problem with its
   need for alpha conversion of an inner lambda abstraction and
   so is preferred in practical graph reduction systems.

   The same principle is often used in strict languages such as
   Scheme to provide call-by-name evaluation by wrapping an
   expression in a lambda abstraction with no arguments:

   	D = delay E = \ () . E

   The value of the expression is obtained by applying it to the
   empty argument list:

   	force D = apply D ()
   		= apply (\ () . E) ()
   		= E

   (1994-10-31)