Result is abs(Number)
which should be preferred for portability.
Number and Result have to be of the same type.
In coroutining mode, if Number is uninstantiated, the call to abs/2 is delayed until this variable is instantiated.
Success:
abs(1, 1).
abs(-5, Result). (gives Result = 5)
abs(-6.2, Result). (gives Result = 6.2)
Fail:
abs(1, 0).
abs(1, 1.0).
abs(-1.0, 1).
abs(1, r).
Error:
abs(A, 6). (Error 4).
abs(4 + 2, 6). (Error 24).