(HNF) A term describing a
lambda expression whose 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.
Compare
Weak Head Normal Form.