hnf_list_head.chr

:- module(hnf_list_head, [first/2, rest/2, kind/2, fixed/2, type(lit/0)]).
:- chr_constraint first/2, rest/2, kind/2, fixed/2.
:- chr_type lit ---> a ; b ; c.

% Head matches cons cell, extracts head.
first([H|_], R) <=> R = H.
first([], R)    <=> R = empty.

% Head matches cons cell, extracts tail.
rest([_|T], R) <=> R = T.
rest([], R)    <=> R = empty.

% Tag the input as empty / cons / other.
kind([], R)     <=> R = empty.
kind([_|_], R)  <=> R = cons.
kind(_, R)      <=> R = other.

% Fixed-length pattern: matches exactly [a, b, c].
fixed([a, b, c], R) <=> R = abc.
fixed(_, R)         <=> R = no_match.