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From: Mild Shock <janburse@fastmail.fm>
Newsgroups: comp.lang.prolog
Subject: Mode yfy to the rescue (Was: How Prolog became an education
 nightmare)
Date: Thu, 14 Nov 2024 22:30:11 +0100
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Ok, here is my minimal test case, to
demonstrate that fy 2 yf has not much
practical use. Irrespective how it is

parsed. The test case is the computer
science professor who wants to teach
compiler construction, and defines a

toy language that has among other things
a Pascal begin end:

:- op(1100, fy, begin).
:- op(1100, yf, end).

Neither SWI-Prolog nor SICStus Prolog can parse this:

?- X = (begin x=1 end; begin y=2 end).
ERROR: Syntax error: Operator priority clash

I am also not able to parse it in Dogelog
Player, Trealla Prolog and Scryer Prolog.
I guess it has to do that disjunction uses

xfy mode. Mostlikely yfx mode would also not
help? What would help if there were
a yfy mode (sic!).


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?
>