Re: Ann: SWI-Prolog 7.1.0

On 11/22/2013 06:53 PM, Carlo Capelli wrote:
> Hi Jan
> 
> There must be something complex I don't understand
> 
> 1 ?- phrase("hello","hello").
> 
> ERROR: Type error: `list' expected, found `hello'

Yes.  Although "..." can be used as a _terminal_ in DCGs, which causes
it to be translated into a code list, it is not valid input or output
of a DCG.  This is not a big issue in applications as the input of a
DCG generally does not appear in the source code, but comes from some
reading predicate.

What works is this:

?- phrase("hello",`hello`).
true

or

?- phrase(`hello`,`hello`).
true

`...` is now read as a code list.  This is not yet `official' (it is
not even documented).  I'm getting more and more convinced that this
is the right decision.

> Also this behaviour is unexpected: I understood from docs that DCGs
> literals are already handled...
> Maybe I misunderstood documentation
> <http://www.swi-prolog.org/pldoc/man?section=ext-dquotes-motivation> and
> it's mandatory to set the flag to get old code running ?

Typically not, as explained above.  You only need it if you do a lot of
general list processing that takes double quoted strings as arguments.
E.g., append(Codes, "!", Result).  There you either need `...`, use
something else to create a code list (i.e., string_codes("!", Exclamation))
or change the Prolog flag for the module.

	Cheers --- Jan