2.7.13.2 Filter

Another well known list operation is that which applies a predicate to every item of a list, and deletes those for which the predicate is false. For a predicate with virtual code p, such an application can be coded conveniently in this form,

T23

[[filter]] p = ((nil,nil),(nil,(p,nil)))

which is to say that writing ((nil,nil),(nil,(p,nil))) in silly is the same as writing filter p.

The virtual machine detects code of this form provided that p is other than nil, and evaluates it consistently with the following properties, causing it to have the meaning that it does.

P26

([[filter]] p) nil = nil

P27

([[filter]] p) (x,y) = ([[filter]] p) y if p x = nil

P28

([[filter]] p) (x,y) = (x,([[filter]] p) y) if p x is a non-nil tree