catflap.org Online Dictionary Query

  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)