Result is sqrt(Number)
which should be preferred for portability.
In coroutining mode, if Number is uninstantiated, the call to sqrt/2 is delayed until this variable is instantiated.
Success:
sqrt(1.0, 1.0).
sqrt(1.0, Result). (gives Result = 1.0)
sqrt(49, Result). (gives Result = 7.0)
Fail:
sqrt(1, 0.0).
sqrt(1, 1).
sqrt(1, r).
Error:
sqrt(A, 6.0). (Error 4).
sqrt(-2, Result). (Error 20).
sqrt(4 + 2, 2.44949). (Error 24).