Fsyntax in swipl

General
1

hi so i tried Ciao, and it has a module called fsyntax, which lets you use the last argument of predicates as a “return value”

is there such a modules for SWI? would it be possible to write one?

1 Like
2

The fsyntax looks handy. This does remind me of the func pack available for swi-prolog, but honestly the syntax and documentation is a little cryptic to me. However I think that library is suppose to provide similar functionality. The $ syntax is something I’ve seen in R as well.

For some cases a DCG could be all you need, since

chain --> first, second, third.

is literally the same as

chain(A, B) :-
    first(A, C),
    second(C, D),
    third(D, B).
1 Like
3

Sure. SWI-Prolog already has the Dict.Func functions and same infra structure can be reused to define other constructs that act as function. It is mainly a matter of choosing a syntax. The func pack is one. I don’t think it uses the low-level functional notation syntax support as that probably did no exist back then.

1 Like
4

how could one do it? would it be via term_expansion/2?

5

See boot/expand.pl. It might be enough to define extra clauses for the predicate '$expand':function/2, where the first is the term that must be interpreted as a function and the second is (I think) the module in which context the expansion takes place. So, I one could do

:- multifile '$expand':function/2.
'$expand':function($(_), _).

$(Term, Result) :- call(Term, Result).

And now this works :slight_smile:

?- member(X, $append([1,2], [3])).
X = 1 ;
X = 2 ;
X = 3.
2 Likes
6

wow, there already is a syntax in SWI. thank you

7

i also noticed that := is already an operator. however, using it just… defines a predicate named (:=)/2

?- [user].
|: says(cat) := meow.
|: says(dog) := woof.
|: ^D% user://1 compiled 0.01 sec, 2 clauses
true.

?- says(cat, What).
ERROR: Unknown procedure: says/2 (DWIM could not correct goal)

?- :=(says(cat), What).
What = meow.
8

i did something… not sure how correct is this tho

functor_from_to_args(From-To, From, To, []).
functor_from_to_args(term_position(_, _, From, To, Args), From, To, Args).
fun_pos(
	term_position(From, To, _, _, [HeadPos, ResultPos]),
	term_position(From, To, FunctorFrom, FunctorTo, ArgsResult)
) :- functor_from_to_args(HeadPos, FunctorFrom, FunctorTo, Args), append(Args, [ResultPos], ArgsResult).
user:term_expansion(F := Result, T0, P, T1) :-
	F =.. F_, append(F_, [Result], P_), P =.. P_,
	( fun_pos(T0, T1) -> true ; true ).
9

Operators have no relation to functions/functional notation. They are just syntactic sugar, transforming e.g. 1+2 into +(1,2). + in Prolog in general does not mean addition. Only when used as argument to is/2, >/2 and similar arithmetic predicate, these predicates interpret the + as addition.

Great, together with a bunch of code gives us little option to comment :slight_smile: What did you try to achieve and how did it (not) work?

10

it adds the part after := as an argument:

?- [user].
|: not(no) := yes.
|: not(yes) := maybe_later.
|: ^D% user://3 compiled 0.00 sec, 2 clauses
true.

?- listing(not/2).
not(no, yes).
not(yes, maybe_later).