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Weak Head Normal Form
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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
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.