Re: How Prolog became an education nightmare (Was: 50 Years of Prolog Nonsense)

Lets cut through the thicket. There is no
real world use case of a (fy 1 yf). Take again
the Pascal “begin” “end” mixfix example.
Typically we want to then go on and write
for example a compiler:
:- op(1100,fy,begin).
:- op(1100,yf,end).
compile((begin X end)) --> compile(X). %%% scoping omitted
compile((X;Y)) --> compile(X), compile(Y).
compile((V=E)) --> [load(E),store(V)].
The problem is the pattern (begin X end) will
not work, if multiple (begin … end) are involved
in the compile/1 call. You can try yourself, no
Prolog system can do it:
/* SWI-Prolog */
?- X = (begin
           x = 1;
           begin
                y = 2
           end
        end), compile(X, L, []).
false.
%%% expected L = [load(1),store(x),load(2),store(y)]
/* SICStus Prolog */
?- X = (begin
           x = 1;
           begin
                y = 2
           end
        end), compile(X, L, []).
no.
%%% expected L = [load(1),store(x),load(2),store(y)]
The reason is that the parser will join multiple yf,
similarly it would join multiple fy. The parser will
not follow a braket pattern. At least I don’t know
any Prolog system that can do it.
Mild Shock schrieb:

Here is the SWI-Prolog and the SICStus
Prolog abstract parse term. This is the real
nightmare of every computer science professor,
 who wants to use Prolog in a compiler
construction course:
 /* SWI-Prolog 9.3.14 */
end(end(begin(;(=(x,1),;(=(y,2),begin(=(z,3)))))))
 /* SICStus Prolog 4.9.0 */
begin(;(=(x,1),;(=(y,2),begin(end(end(=(z,3)))))))
 On the other hand mostlikely the OP @horsh
would expect:
 /* What the End-User wants */
end(begin(;(=(x,1),;(=(y,2),end(begin(=(z,3)))))))
 I think its impossible to do in any Prolog system,
you would need a programming language with the
possibility to do mixfix syntax definitions,
 like for example in Isabelle/HOL:
 (* Define the mixfix syntax for a Pascal-like block *)
syntax
   "_begin_end" :: "'a ⇒ 'a" ("begin _ end")
 Or otherwise use DCG to write your own parser
for the DSL at hand, that you want to parse.
Since Prolog operators cannot model mixfix syntax,
 at least SWI-Prolog and SICStus Prolog fail,
and I guess other Prolog systems fail as well.
 Mild Shock schrieb:

Concerning the input (xxx yyy zzz) the OP wrote:
>
I would expect it to print zzz(xxx(yyy)).
>
Where did he get this requirement from, he didn’t
compare other Prolog systems, right? So it came from
his applicationdomain. But what was his application
>
domain? Ok, lets proceed to an example with multiple
brakets. Lets make the Pascal “begin” “end” example,
by replacing xxx and zzz by “begin” and “end”.
>
I get this result:
>
?- member(X,[begin,end]), current_op(Y,Z,X).
X = (begin), Y = 1100, Z = fy ;
X = (end), Y = 1100, Z = yf.
>
?- X = (begin
|          x = 1;
|          y = 2;
|          begin
|               z = 3
|          end
|       end).
X = (begin x=1;y=2;begin z=3 end end).
>
But is the abstract parse term, the Prolog result useful?
>