Semicolon and parentheses in clause body

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Ferenc Nagy
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Semicolon and parentheses in clause body

Unread post by Ferenc Nagy » 11 Jan 2016 13:23

Hi,
Here is code fragment containing bla-bla(branch-1;branch2 ) finish1;catch_all :

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predicates     onMouseDown : window::mouseDownListener. % Original clause clauses     onMouseDown(_Source, Point, _ShiftControlAlt, _Button) :-        Point = pnt(X0,Y0),        X = X0-x, Y = Y0-d,        X>0, Y>0,        X<field, Y<field,!,        I = X div cell_size,        J = Y div cell_size,        (         cell(step,I,J),         retract(cell(step,I,J)),         aliveN := aliveN-1,         showXY(I,J,empty),!;         assert(cell(step,I,J)),         aliveN := aliveN+1,         showXY(I,J,life)        ),        integerControl1_ctl:setInteger(aliveN);        succeed.
Is the above code a shortening of three statements below?

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%1  onMouseDown(_Source, Point, _ShiftControlAlt, _Button) :-        Point = pnt(X0,Y0),        X = X0-x, Y = Y0-d,        X>0, Y>0,        X<field, Y<field,!,        I = X div cell_size,        J = Y div cell_size,         cell(step,I,J),         retract(cell(step,I,J)),         aliveN := aliveN-1,         showXY(I,J,empty),!,        integerControl1_ctl:setInteger(aliveN),!. %2 onMouseDown(_Source, Point, _ShiftControlAlt, _Button) :-        Point = pnt(X0,Y0),        X = X0-x, Y = Y0-d,        X>0, Y>0,        X<field, Y<field,!,        I = X div cell_size,        J = Y div cell_size,         assert(cell(step,I,J)),         aliveN := aliveN+1,         showXY(I,J,life)        integerControl1_ctl:setInteger(aliveN),        !. %3 onMouseDown(_Source, Point, _ShiftControlAlt, _Button) :-        succeed.
The original clause is indeed shorter than the 3-clause version but it is not transparent for me.
I remember from the manual that ";" means "or" but I do not remember such usage of pairs of parentheses within a clause body.
TIA, Regards,
Frank Nagy

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Thomas Linder Puls
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Unread post by Thomas Linder Puls » 11 Jan 2016 21:39

and (,) has higher precedence than or (;).

Parentheses are used to circumvent/control the precedence.

So

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A and B or C and D
is the same as

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(A and B) or (C and D)
or

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AB or CD where     AB is A and B     CD is C and D
On the other hand

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A and (B or C) and D
is

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A and BorC and D where     BorC is B or C
Regards Thomas Linder Puls
PDC

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Ferenc Nagy
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When do you use more clauses and when do you use semicolons?

Unread post by Ferenc Nagy » 12 Jan 2016 8:05

Thomas,
What is your practice? When do you use separated clauses for the same predicate, and when do you merge the clauses into an single clause containing and+or operators?

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clauses   % Written as two clauses   alfa(A,B,_,_) :-   A, B. alfa(_,_,C,D) :-   C, D.   % Merged in a single clause using and+or alfa(A,B,C,D) :-  (A and B) or (C and D).   % Merged in a single clause using command and semicolon alfa(A,B,C,D) :-  (A,B) ; (C,D).  
When do you recommend the first coding style, and when shall be the second and third coding lookouts better?
TIA, Regards,
Frank Nagy

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Thomas Linder Puls
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Unread post by Thomas Linder Puls » 12 Jan 2016 9:58

I would never use an outermost or:

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p :- A or B.
in favor of

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p :- A. p :- B.
But if the or is not on outermost level, I could use is:

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p :- A, (B or C), D.
Then it can be preferable to repeating code:

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p :- A, B, D. p :- A, C, D.
But in many/most cases I will actually prefer an auxiliary predicate:

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p :- A, q, D. q :- B. q :- C.
In general, I use or rarely, and I mainly use it for two purposes:
  • A logical operator in a condition (but here the I often use orelse instead)
  • A nondeterministic choice point
Logical operator examples (I actually prefer orelse here):

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if A < 10 or A > 100 then ...   foreach X in L and (X < 10 or X > 100) do ...

Nondeterministic choice point examples

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clauses     p(....) = R :-         ... % lot of calculation         (R = A or R = B). % two results (nondeterministic)   foreach X in L1 or X in L2 or X = 17 do ...
Furthermore, I always write or in favor of ;, because the difference between or and and is much more significant than the graphical difference between ; and ,.
Regards Thomas Linder Puls
PDC

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Ferenc Nagy
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Or vs. orelse

Unread post by Ferenc Nagy » 12 Jan 2016 11:56

Thank you, Thomas:
The expression

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a(...) or b(...)
always evaluates b(...) which is in general needless if a(...) succeeds and may result a runtime error within b(...). The evaluation of b(...) is necessary after successful a(...) only if a side-effect of b(...) [e. g. setting a fact like b_result:=true or b_result:=false] is required for later usage.
The expression

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a(...) orelse b(...)
always evaluates b(...) only if b(...) fails so it saves time and avoids some runtime errors.
TIA, Regards,
Frank Nagy

Paul Cerkez
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Unread post by Paul Cerkez » 12 Jan 2016 13:25

my half cent perspective:

I never use the logical operators AND, OR within a clause to execute two different actions/results as they can be really problematic when trouble shooting.

I always use extra clauses. (call it a personal preference :-) )

in the case of a(...) OR b(...) I would use what Thomas describes
I would never use an outermost or:

p :- A or B.

in favor of

p :- A.
p :- B.

But if the or is not on outermost level, I could use is:

p :- A, (B or C), D.

Then it can be preferable to repeating code:

p :- A, B, D.
p :- A, C, D.
If a(...) succeeds, b(...) never gets evaluated. If a(...) fails, the b(...) is active. Any code that may be duplicated in a(...) and b(...) I just put into a 'sub clause' if it is more than a few lines.

p(...):- a(..., p1, ...), !.
p(...):- b(..., p1, ...), !.
p1(...):- %common code

********************
At one time, it was also explained that when the compiler created the object code, it effectively treated the embedded ORs as separate clauses anyway. That was back in 5.x, don't know if it still works that way in 7.x

P.
AI Rules!
P.

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Thomas Linder Puls
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Re: Or vs. orelse

Unread post by Thomas Linder Puls » 12 Jan 2016 15:03

Ferenc Nagy wrote:Thank you, Thomas:
The expression

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a(...) or b(...)
always evaluates b(...) which is in general needless if a(...) succeeds and may result a runtime error within b(...).
That is fortunately not true.

The evaluation of A or B consist of creating a backtrack point to B and then evaluating A. B will be evaluated only when/if backtracking to it. orelse removes the backtrack point if A succeeds, so in an orelse failure in A will backtrack to B, but A orelse B does not leave a backtrack point after the construction.
Regards Thomas Linder Puls
PDC

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Ferenc Nagy
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Shortcut logical operators in other programming languages

Unread post by Ferenc Nagy » 13 Jan 2016 8:05

The new Visual Basic contains the orelse operator https://msdn.microsoft.com/hu-HU/library/ea1sssb2.aspx.
If either or both expressions evaluate to True, result is True. The following table illustrates how result is determined.
This language does not know the backtracking which is the unique feature of all Prolog versions.

When I was learning the Fortran language in the 1970-s, its manuals did not contain whether the B function is called if the result of determines the

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EAND=A(X,Y).AND.B(X,Y)
and the

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EOR=A(X,Y).OR.B(X,Y)
expressions, respectively.
The later FORTRAN90 had compiler options allowing xor prohibiting the partial evaluation of logical expressions. I wrote the program http://franknagy.atw.hu/Angol/Beatrice.htm using FORTRAN 90 during the years 2003..2005 .
Attachments
Vb orelse.PNG
Table of truth of "orelse" operator of the visual Basic Language.
Vb orelse.PNG (7.79 KiB) Viewed 4247 times
TIA, Regards,
Frank Nagy

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