Weak Head Normal Form
<reduction, lambda calculus> (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)
Nearby terms:
WDASM « WDM « WE « Weak Head Normal Form »
weakly typed » weak typing » weasel
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