yason has problem encoding "pure" cons

[jcguu95]

Yason does not encode a lisp pure cons (a pure cons is a cons whose cdr is not a cons) into json correctly. For example, an attempt to encode (1 . 2) returns the error.

* (ql:quickload :yason)
* (yason:encode (cons 1 2) *standard-output*)
#<THREAD "main thread" RUNNING {7008680273}>:
  The value
    2
  is not of type
    LIST

This is because (defmethod encode ((object list) ..) treats "pure cons" as lists too,
which seems to be a "feature" of common lisp.

* (defmethod test ((object list)) (pprint object))
#<STANDARD-METHOD COMMON-LISP-USER::TEST (LIST) {7005484C83}>
* (test (cons 1 2))
(1 . 2)

Solutions:

1. Make it more explicit that "pure cons" are not supported.
2. Support encoding pure cons.

[phmarek]

Well, yeah; you're right.

But it's not just plain cons cells -- what should we do for '(1 2 . 3)? Walk the entire input list to decide whether it's a proper list or not, and switch behaviour? Or just serialize '(1 2 . 3) the same as '(1 2 3)?

[hanshuebner]

If you represent your data as lists or cons trees, I'd recommend that you use a custom serialization function instead of trying to guess the actual type of the cons that is being serialized.