Terminology:
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Abbreviations

Thunk:
\_:Unit.t  - thunked form of t

Jones - STG...
The remaining lambda-forms, those with an empty argument list but not of the
special form of a PAP or a c-tor, are called thunks.

CAF - constant applicative form
is a global THUNK object (static heap-obj).
Other TUNKS are local in let-expressions of STG.

Lambda-lifting:
Each lambda-form has either
- no free variables [supercombinator] - values held in the just-entered closure
- no arguments [thunk] - values on the stack

Lambda-lifted program has only supercombinators or thunks


Heap object can be:
- FUN (function definition)
- PAP (partial application)
- CON (completely applied c-tor)
- THUNK
- BLACKHOLE

Stack
- is used for case-expr evaluation.


WHNF, HNF, NF

WHNF = NF
  | starts with lambda
  | the root sym is c-tor (no delta rules for this)
  | the root is a var