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| Weak Head Normal Form (WHNF) A is in weak head normal form (WHNF) if it is a (HNF) or any . I.e. the top level is not a . The term was coined by to make explicit the difference between (HNF) and what 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 problem with its need for of an inner lambda abstraction and so is preferred in practical systems. The same principle is often used in languages such as to provide 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) |
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