Weak head normal form

An expression is in weak head normal form (WHNF), if it is either:

• a constructor (eventually applied to arguments) like True, Just (square 42) or (:) 1.
• a built-in function applied to too few arguments (perhaps none) like (+) 2 or sqrt.
• or a lambda abstraction \x -> expression.

Note that the arguments do not themselves have to be fully evaluated for an expression to be in weak head normal form; thus, while square 42 can be reduced to 42 * 42 (which can itself be reduced to a normal form of 1764), Just (square 42) is in WHNF without further evaluation. Similarly, (+) (2 * 3 * 4) is in WHNF, even though 2 * 3 * 4 could be reduced to the normal form 24.

An exception is the case of a fully applied constructor for a data type with some fields declared as strict; the arguments for these fields then also need to be in WHNF.

The above definition might seem to treat built-in functions differently from functions defined via lambda abstraction. However, the distinction does not matter to semantics. If a lambda abstraction is applied to "too few arguments", then evaluating the application just means substituting arguments for some of the lambda abstraction's variables, which always halts with the result a now-unapplied lambda abstraction.

See also

• The Haskell wikibook
• stackoverflow
• TheFreeDictionary.