theory Geoff Burn defines evaluators E0, E1, E2 and E3 which

when applied to an expression, reduce it to varying degrees.

E0 does no evaluation, E1 it evaluates to weak head normalform (WHNF), E2 evaluates the structure of a list, i.e. it

evaluates it either to NIL or evaluates it to a CONS and then

applies E2 to the second argument of the CONS. E3 evaluates

the structure of a list and evaluates each element of the list

to WHNF. This concept can be extended to data structures

other than lists and forms the basis of the evaluationtransformer style of strictness analysis.