On Tue, Oct 4, 2016 at 5:47 PM, D.R. Buckley, Jr. <delmas.buckley@gmail.com> wrote:

Downloaded and installed the "stable" version (1.14) on Ubuntu and tried to run code that works fine on SWI-Prolog. One predicate used is call/1 and Ciao places that in a library hiord_rt that doesn't appear to be included in the stable version. How do I get and install that library? Or is there a workaround? I want to use it in the following code that adds list comprehensions to Prolog.

%-------------------------------------------------------

%% list comprehension defined

:- op(700, xfx, [ <- ]).

:- op(450, xfx, [ .. ]).

:- op(1100, yfx, [ & ]).

Y <- M .. N :-

integer(M),

integer(N),

M =< N,

between(M, N, Y).

Z <- {Var & Dec & Pred} :-

findall(Var, maplist(call, [Dec, Pred]), Z).

%-------------------------------------------------------

Dear Robert Buckley,

There is no need for combining maplist/2 and call/1. This is a more direct implementation:

Z <- {Var & Dec & Pred} :-

findall(Var, (call(Dec), call(Pred)), Z).

This gives the same results as the example from https://rosettacode.org/wiki/List_comprehensions#Prolog . E.g.,

?- V <- {X, Y, Z & X <- 1..20, Y <- X..20, Z <- Y..20 & X*X+Y*Y =:= Z*Z}.

V = [(3,4,5),(5,12,13),(6,8,10),(8,15,17),(9,12,15),(12,16,20)] ?��

In any case, the code above has some problems. I would not use it except for toy examples. Using "idiomatic" Prolog may result in cleaner and more efficient code. There is a relation between the Haskell list type, lazy evaluation, and Prolog backtracking. findall/3 (or setof/3, bagof/3, etc.) are useful to compute all the solutions of a predicate or goal, which is equivalent of fully evaluate a "list monad" in Haskell. In some sense Prolog does not have list comprehensions because it *already* had them before they were invented.��

This code:

?- V <- {X, Y, Z & X <- 1..20, Y <- X..20, Z <- Y..20 & X*X+Y*Y =:= Z*Z}.

can be written in a more standard style (readable by other Prolog programmers) as:

?- findall((X,Y,Z), (between(1,20,X), between(X,20,Y), between(Y,20,Z), X*X+Y*Y =:= Z*Z), V).

I hope this helps!

Cheers,

--

Jose

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