Scryer-prolog: library(dcgs): missing ('|')//2

[UWN]

[UWN]

?- use_module(library(dcgs)).
true.
?- op(1105,xfy,'|').
true.
?- phrase(("a"|"b"),L).
caught: error(existence_error(procedure,('|')/4),('|')/4)
?- phrase(("a";"b"),L).
L = [a] ;
L = [b].

Ideally, ('|')//2 describes an alternative, but rejects if-then-else during term-to-body conversion.

[mthom]

I don't know how to retain the usual meaning of ('|') as a head-tail separator in lists while overloading it as an operator (or if I should).

[UWN]

(So this is now a syntax question only) There is no ambiguity between head-separator and the infix bar operator, since the infix bar must have a priority of 1001 or higher. See http://www.complang.tuwien.ac.at/ulrich/iso-prolog/dtc2#Res_A78 The elements within a list have always been restricted to See in 6.3.5 the use of non-terminal arg defined in 6.3.3.1 reading:

           arg =  atom ;
Abstract:  a      a
Condition: a is an operator

           arg =  term ;
Abstract:  a      a
Priority:         999

Ambiguity would only arise with smaller priority.

[UWN]

In any case for library(dcgs), within the actual module, the bar must be written in functional notation.

[UWN]

Now its all wrong, one level too much:

?- phrase(("a"|"b"),L).
L = [[a]] ;
L = [[b]].
?- phrase("a",L).
L = [[a]].
?- L = "a".
L = [a].
?- phrase(("a";"b"),L).
L = [[a]] ;
L = [[b]].