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2.3  ERRORS

	An Ada program may be incorrect for various reasons.  Three
categories are recognized according to how they are detected.

     Some errors wil be detected by the compiler - these will include
simple punctuation mistakes such as leaving out a semicolon or attempting
to violate the type rules such as mixing up colours and fish.  In these
cases the program will not be executed.

     Other errors are detected when the program is executed.  An attempt to
find the square root of a negative number or divide by zero are examples of
such errors.  In these cases an exception is raised as we saw in the last
section and we have an opportunity to recover from the situation.

     Finally there are certain situations where the program breaks the
language rules but there is no simple way in which this violation can be
detected.  For example a program should not depend on the order in which
parameters of a procedure call are evaluated.  If it does then the
behaviour may depend upon the implementation.  Such a program is said to be
erroneous.  Care must be taken to avoid writing erroneous programs; in
practice if we avoid clever tricks then all will be well.