post-N288 call_nth/2.

Ulrich Neumerkel (Version history)

p.p.9 call_nth/2

p.p.9.1 Description

Procedurally, call_nth(G_0, Nth) is executed as follows:

a) Creates a counter N with value zero,

b) If Nth is an integer smaller or equal N, the goal fails,

or b') If there is no value greater N that is unifiable with Nth, the goal fails,

c) (Re-)executes call(G_0),

d) If it fails, the goal fails,

e) Else if it succeeds, sets N to N+1,

f) Unifies Nth with counter N,

g) If the unification succeeds, the goal succeeds,

h) Else proceeds to p.p.9.1 b,

p.p.9.2 Template and modes

p.p.9.3 Errors

a) G_0 is a variable and Nth is not zero
— instantiation_error.

b) G_0 is neither a variable nor a callable term
and Nth is not zero
— type_error(callable, G_0).

c) Nth is neither a variable nor an integer
— type_error(integer, Nth).

d) Nth is an integer less than zero
— domain_error(not_less_than_zero, Nth).

e) The value of flag bounded is true and at step
p.p.9.1 e N+1 is larger than max_integer
— evaluation_error(int_overflow).
—representation_error(max_integer).

p.p.9.4 Examples

?- call_nth(true, Nth).
   Nth = 1.

?- call_nth(false, Nth).
   false.

?- call_nth(repeat, Nth).
   Nth = 1
;  Nth = 2
;  Nth = 3
;  Nth = 4
;  Nth = 5
;  ... .

?- call_nth(( N = 1 ; N = 2 ), Nth).
   N = 1, Nth = 1
;  N = 2, Nth = 2.

?- call_nth(true, non_integer).
   type_error(integer,non_integer).

?- call_nth(true, 1.0).
   type_error(integer,1.0).

?- call_nth(true, 0).
   false.

?- call_nth(repeat, 0).
   false.

?- call_nth(repeat, -1).
   domain_error(not_less_than_zero,-1).

?- call_nth(length(L,N), 3).
   L = [_A,_B], N = 2.

?- call_nth(inex, 0).
   false. % thus not existence_error(procedure,inex/0)

?- call_nth(inex, 0).
   existence_error(procedure,_), unexpected.

?- call_nth(1, 0).
   false.

?- call_nth(V, 0).
   false.

?- call_nth(N = 1, N).
   N = 1.

?- call_nth(N = -1, N).
   false.

?- call_nth(repeat,1+1).
   type_error(integer,1+1).

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