constant applicative form
<functional programming> (CAF) A supercombinator which is not a lambda
abstraction. This includes truly constant expressions such as 12, (+ 1 2), [1,
2, 3] as well as partially applied functions such as (+ 4). Note that this last
example is equivalent under eta abstraction to \ x . + 4 x which is not a CAF.
Since a CAF is a supercombinator, it contains no free variables. Moreover, since
it is not a lambda abstraction it contains no variables at all. It may however
contain identifiers which refer to other CAFs, e.g.
c 3 where c = (* 2).
A CAF can always be lifted to the top level of the program. It can either
be compiled to a piece of graph which will be shared
by all uses or to some shared code which will
overwrite itself with some graph the first time it
is evaluated. A CAF such as
ints = from 1 where from n = n : from (n+1)
can grow without bound but may only be accessible from within the code of
one or more functions. In order for the garbage
collector to be able to reclaim such structures, we
associate with each function a list of the CAFs to
which it refers. When garbage collecting a reference
to the function we collect the CAFs on its list.
[The Implementation of Functional Programming Languages, Simon Peyton Jones].
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