AT4WS09 slide-19

No Wishful Thinking

Guessing without knowing may be futile.

Consider web service Withdraw.

- Has "balance" as input and output.
- "Amount" to be withdrawn is a known input.
- "Balance" is not known until runtime.
- Precondition is that Balance > Withdrawal Amount.

- Whether that should be assumed may be determined by case.
But Balance should not be Selectable. If Balance is required to be an exact amount, and there is no way to make a sub-plan to make it so, guessing that it is so is just futile.