Error handling techniques

This chapter describes various error handling techniques. First the technique is described, then its use is shown with an example function and a call to that function. We use the √ function which should report an error condition when called with a negative parameter.

To be specific: Exceptions is the Ada way to go.

Return code

procedure Error_Handling_1 is

function Square_Root (X : in Float) return Float is
begin
if X < 0.0 then
return -1.0;
else
return Sqrt (X);
end if;
end Square_Root;

begin

C := Square_Root (A ** 2 + B ** 2);

if C < 0.0 then
T_IO.Put ("C cannot be calculated!");
else
T_IO.Put ("C is ");
F_IO.Put
(Item => C,
Fore => F_IO.Default_Fore,
Aft  => F_IO.Default_Aft,
Exp  => F_IO.Default_Exp);
end if;

end Error_Handling_1;

Our example makes use of the fact that all valid return values for √ are positive and therefore -1 can be used as an error indicator. However this technique won't work when all possible return values are valid and no return value is available as error indicator; hence to use this method is a very bad idea.

Error (success) indicator parameter

An error condition is returned via additional out parameter. Traditionally the indicator is either a boolean with "true = success" or an enumeration with the first element being "Ok" and other elements indicating various error conditions.

procedure Error_Handling_2 is

procedure Square_Root
(Y       : out Float;
X       : in  Float;
Success : out Boolean)
is
begin
if X < 0.0 then
Y       := 0.0;
Success := False;
else
Y       := Sqrt (X);
Success := True;
end if;
return;
end Square_Root;

begin

Square_Root
(Y       => C,
X       => A ** 2 + B ** 2,
Success => Success);

if Success then
T_IO.Put ("C is ");
F_IO.Put (Item => C);
else
T_IO.Put ("C cannot be calculated!");
end if;

end Error_Handling_2;

One restriction for Ada up to Ada 2005 is that functions cannot have out parameters. (Functions can have any side effects but may not show them). So for our example we had to use a procedure instead. The bad news is that the Success parameter value can easily be ignored.

In Ada 2012, functions may have parameters of any mode; hence this is possible at last:

function Square_Root
(X       : in  Float;
Success : out Boolean) return Float
is
...

This technique does not look very nice in mathematical calculations; hence no good idea either.

Global variable

An error condition is stored inside a global variable. This variable is then read directly or indirectly via a function.

procedure Error_Handling_3 is

Float_Error : Boolean;

function Square_Root (X : in Float) return Float
is
begin
if X < 0.0 then
Float_Error := True;
return 0.0;
else
return Sqrt (X);
end if;
end Square_Root;

begin

Float_Error := False;  -- reset the indicator before use
C := Square_Root (A ** 2 + B ** 2);

if Float_Error then
T_IO.Put ("C cannot be calculated!");
else
T_IO.Put ("C is ");
F_IO.Put
(Item => C,
Fore => F_IO.Default_Fore,
Aft  => F_IO.Default_Aft,
Exp  => F_IO.Default_Exp);
end if;

end Error_Handling_3;

As you can see from the source, the problematic part of this technique is choosing the place at which the flag is reset. You could either have the callee or the caller do that.

Also this technique is not suitable for multithreading.

Use of global variables for cases like this indeed is a very bad idea, in effect one of the worst.

Exceptions

Ada supports a form of error handling that has long been used by other languages like the classic ON ERROR GOTO ... from early Basic dialects to the try ... catch exception handling from modern object oriented languages.

The idea is: You register some part of your program as error handler to be called whenever an error happens. You can even define more than one handler to handle different kinds of errors separately. Once an error occurs, the execution jumps to the error handler and continues there; it is impossible to return to the location where the error occurred.

procedure Error_Handling_4 is

Root_Error: exception;

function Square_Root (X : in Float) return Float is
begin
if X < 0.0 then
raise Root_Error;
else
return Sqrt (X);
end if;
end Square_Root;

begin

C := Square_Root (A ** 2 + B ** 2);

T_IO.Put ("C is ");
F_IO.Put
(Item => C,
Fore => F_IO.Default_Fore,
Aft  => F_IO.Default_Aft,
Exp  => F_IO.Default_Exp);
exception
when Root_Error =>
T_IO.Put ("C cannot be calculated!");

end Error_Handling_4;

The great strength of exceptions handling is that it can block several operations within one exception handler. This eases the burden of error handling since not every function or procedure call needs to be checked independently for successful execution.

Design by contract

In Design by Contract (DbC), operations must be called with the correct parameters. This is the caller's part of the contract. If the subtypes of the actual arguments match the subtypes of the formal arguments, and if the actual arguments have values that make the function's preconditions True, then the subprogram gets a chance to fulfill its postcondition. Otherwise an error condition occurs.

Now you might wonder how that is going to work. Let's look at the example first:

procedure Error_Handling_5 is

subtype Square_Root_Type is Float range 0.0 .. Float'Last;

function Square_Root
(X    : in Square_Root_Type)
return Square_Root_Type
is
begin
return Sqrt (X);
end Square_Root;

begin

C := Square_Root (A ** 2 + B ** 2);

T_IO.Put ("C is ");
F_IO.Put
(Item => C,
Fore => F_IO.Default_Fore,
Aft  => F_IO.Default_Aft,
Exp  => F_IO.Default_Exp);

return;
end Error_Handling_5;

As you can see, the function requires a precondition of X >= 0 — that is, the function can only be called when X ≥ 0. In return the function promises as postcondition that the return value is also ≥ 0.

In a full DbC approach, the postcondition will state a relation that fully describes the value that results when running the function, something like result ≥ 0 and X = result * result. This postcondition is √'s part of the contract. The use of assertions, annotations, or a language's type system for expressing the precondition X >= 0 exhibits two important aspects of Design by Contract:

1. There can be ways for the compiler, or analysis tool, to help check the contracts. (Here for example, this is the case when X ≥ 0 follows from X's subtype, and √'s argument when called is of the same subtype, hence also ≥ 0.)
2. The precondition can be mechanically checked before the function is called.

The 1st aspect adds to safety: No programmer is perfect. Each part of the contract that needs to be checked by the programmers themselves has a high probability for mistakes.

The 2nd aspect is important for optimization — when the contract can be checked at compile time, no runtime check is needed. You might not have noticed but if you think about it: $A^{2}+B^{2}$ is never negative, provided the exponentiation operator and the addition operator work in the usual way.

We have made 5 nice error handling examples for a piece of code which never fails. And this is the great opportunity for controlling some runtime aspects of DbC: You can now safely turn checks off, and the code optimizer can omit the actual range checks.

DbC languages distinguish themselves on how they act in the face of a contract breach:

1. True DbC programming languages combine DbC with exception handling — raising an exception when a contract breach is detected at runtime, and providing the means to restart the failing routine or block in a known good state.
2. Static analysis tools check all contracts at analysis time and demand that the code written in such a way that no contract can ever be breached at runtime.

Ada 2012 has introduced pre- and postcondition aspects.