guideline

Use short-circuit forms of the logical operators to specify the order of conditions when the failure of one condition means that the other condition will raise an exception.

Use:

if Y /= 0 or else (X/Y) /= 10 then

or:

if Y /= 0 then
   if (X/Y) /= 10 then

rather than either:

if Y /= 0 and (X/Y) /= 10 then

or:

if (X/Y) /= 10 then

to avoid Constraint_Error.

Use:

if Target /= null and then Target.Distance < Threshold then

rather than:

if Target.Distance < Threshold then

to avoid referencing a field in a nonexistent object.

rationale

The use of short-circuit control forms prevents a class of data-dependent errors or exceptions that can occur as a result of expression evaluation. The short-circuit forms guarantee an order of evaluation and an exit from the sequence of relational expressions as soon as the expression's result can be determined.

In the absence of short-circuit forms, Ada does not provide a guarantee of the order of expression evaluation, nor does the language guarantee that evaluation of a relational expression is abandoned when it becomes clear that it evaluates to False (for and) or True (for or).

notes