head normal form

(HNF) A lambda expression is in head normal form if its top

level is either a variable, a data value, a built-in function

applied to too few arguments or a lambda abstraction whose

body is not reducible. I.e. the top level is neither a redex

nor a lambda abstraction with a reducible body.

An expression in HNF may contain redexes in argument postions

whereas a normal form may not. See also Weak head normal

form.