SWI-Prolog -- API-function PL_skip

Documentation
- Reference manual
  - Foreign Language Interface
    - The Foreign Include File
      - Analysing Terms via the Foreign Interface
        
        Reading a list
        
        PL_get_list()
        
        PL_get_head()
        
        PL_get_tail()
        
        PL_get_nil()
        
        PL_skip_list()
- Packages

Availability:C-language interface function

int PL_skip_list(term_t +list, term_t -tail, size_t *len)

This is a multi-purpose function to deal with lists. It allows for finding the length of a list, checking whether something is a list, etc. The reference tail is set to point to the end of the list, len is filled with the number of list-cells skipped, and the return value indicates the status of the list:

PL_LIST: The list is a‘proper' list: one that ends in the list terminator constant and tail is filled with the terminator constant.
PL_PARTIAL_LIST: The list is a‘partial' list: one that ends in a variable and tail is a reference to this variable.
PL_CYCLIC_TERM: The list is cyclic (e.g. X = [a|X]). tail points to an arbitrary cell of the list and len is at most twice the cycle length of the list.
PL_NOT_A_LIST: The term list is not a list at all. tail is bound to the non-list term and len is set to the number of list-cells skipped.

It is allowed to pass 0 for tail and NULL for len.