Error while using the write predicate in Prolog

Question

I was exploring the write predicate in Prolog but it behaves differently at times. I did go through some posts but I am unable to spot the catch.

Predicate :

explore_write_predicate(InputList,Index):-
   TempIndex is Index+1,
   select(Element,InputList,TempList),
   write(TempIndex).

query:

explore_write_predicate([1,2,3,4],1).

Result :

2
true

The above code works fine but when I add one more argument to the write predicate(Element) it gives an error.

Predicate :

explore_write_predicate(InputList,Index):-
   TempIndex is Index+1,
   select(Element,InputList,TempList),
   write(TempIndex,Element).

query:

   explore_write_predicate([1,2,3,4],1).

Error:

No permission to call sandboxed `write(_1132,_1134)'
Reachable from:
      explore_write_predicate(A,B)
      swish_trace:swish_call(explore_write_predicate([1,2,3,4],1))
      '$swish wrapper'(explore_write_predicate([1,2,3,4],1),A)

Please help me why this anomaly. P.S I also saw the documentation for write but could not get much out of it. Any help is greatly appreciated.

Write only has an arity of 1. You need to call it as write([TempIndex,Element]) to make it work. — Enigmativity

Jim Ashworth Jim Ashworth · Accepted Answer · 2017-02-17T10:11:24

Your error is two-fold:

Primarily, you appear to be using SWISH as your interpreter of choice, which locks away a number of input/output methods (including tab/1, write/2, get_single_char/1, put/1, etc.), so you will be unable to use them.

Secondly, write/2 requires a stream as its first argument, to which the second argument will be written - as mentioned in a comment, to write multiple things either pass the items as a list, or use multiple writes.

Error while using the write predicate in Prolog

1 Answers