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Function **/2
Availability:Arithmetic function (see is/2)
[ISO]+Expr1 ** +Expr2
Result = Expr1**Expr2. The result is a float, unless SWI-Prolog is compiled with unbounded integer support and the inputs are integers and produce an integer result. The integer expressions 0 ** I, 1 ** I and -1 ** I are guaranteed to work for any integer I. Other integer base values generate a resource error if the result does not fit in memory.

The ISO standard demands a float result for all inputs and introduces ^/2 for integer exponentiation. The function float/1 can be used on one or both arguments to force a floating point result. Note that casting the input result in a floating point computation, while casting the output performs integer exponentiation followed by a conversion to float.

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Wouter Beek said (2017-10-28T06:25:56):0 upvotes 0 0 downvotes
Picture of user Wouter Beek.

=Notice that the precedence of (**)/2 is xfx, which is different from that of (^)/2, which is xfy.

This may lead to surprises such as the following succeeding:

?- Y = 2, X is 2 ^ -Y.
Y = 2,
X = 0.25.

while the following throws an exception:

?- Y = 2, X is 2 ** -Y.
ERROR: Syntax error: Operator priority clash
ERROR: Y = 2, X is 2 *
ERROR: ** here **
ERROR: * -Y .

The introduction of brackets in the exponent of (**)/2 solves this.=