# Raising and Handling Warnings and Errors

Errors and warnings

During the development and deployment of an AIMMS application, unexpected, possibly harmful, situations can arise. These situations are divided into errors and warnings. An error is a situation that cannot be handled by the procedure encountering it. A warning is a situation that can be handled by the procedure encountering it, but might warrant further inspection by the model developer or by the model user. Note that, even when a procedure cannot handle an error itself, it should be able to recover from that error. In this section, you will find AIMMS facilities to

• handle errors; to handle an error, AIMMS will give you access to the information therein. A handler is a piece of AIMMS code that handles selected errors and warnings. Errors and warnings can be communicated to handlers higher in the execution stack.

• raise an error; not only AIMMS may detect situations warranting an error or warning message, but also the application itself. For such situations AIMMS provides a facility to raise custom errors from within your model.

• handle a legacy situation; external and intrinsic AIMMS procedures may return a status code indicating success or failure. Whenever a failure status of an external and intrinsic procedure remains unnoticed, AIMMS can automatically raise an error in such situations.

• extensively check the code; AIMMS can check your application for many different kinds of situations that occassionally warrant a warning. It is usually worthwhile to apply all these checks to your application.

Tip

You may refer to our online training about error handling following this link.

## Handling Errors

Subsection overview

In this subsection you will find an introduction to both the global and local error handling mechanisms available in AIMMS. Global error handling, by means of specifying a single handler procedure, is used to treat runtime errors occuring anywhere inside the entire model that are not handled elsewhere. Local error handling, by means of the OnError clause in a BLOCK statement, allows error handling of runtime errors occuring in a specific block of code. Global and local error handling are the blocks on which the error handling framework in AIMMS is built. At the end of this subsection, you will find a description of all the intrinsic functions available for accessing and manipulating information regarding errors.

Global error handling

To activate global error handling, the name of a handling procedure in your model must be assigned to the option Global_error_handler. Such a procedure must have a single element parameter argument err in the predeclared set errh::PendingErrors. The global error handling procedure will be executed for each pending error whenever an execution run has been terminated because of errors that have not been handled elsewhere in the model. The global error handler will also be called at the end of a finished execution run if there are unhandled warnings. In this context, an execution run is any call to an AIMMS procedure initiated either through the AIMMS GUI or through the AIMMS API.

Example

Below a global error handling procedure MyErrorHandler is illustrated. The lines in the body of the procedure are numbered to facilitate the explanation of the example.

Procedure MyErrorHandler {
Arguments  :  err;
ElementParameter  err {
Range      :  errh::PendingErrors;
Property   :  Input;
}
Body: {
1  if errh::Node(err) = 'DefP' then
2      DialogMessage(errh::Message(err) + "; resetting P to its default.");
3      Empty P ;
4      errh::MarkAsHandled(err);
5  elseif errh::InsideCategory(err,'IO') then
7         + errh::Message(err)  ); ! Pass adapted message on to next handler.
8  else
9      ! Errors not handled will be passed on to the error/warning window.
10  endif
}
}


Example explanation

The procedure starts with declaring the argument err as an element parameter with the predeclared set errh::PendingErrors, with a subset of the predeclared set Integers as its range. During an execution run, this set is filled with the numbers of the errors and warnings raised. Each number refers to an error or warning with various pieces of information therein, such as its error description, the node in which the error or warning occurred and its severity. In addition, each error belongs to a category. All this information can be accessed using intrinsic functions. The body of the procedure is now explained line by line:

line 1

The intrinsic function errh::Node is used to determine whether or not the error occurred inside the procedure DefP. This intrinsic function returns the identifier or node in which the error occured as an element of the predeclared set AllSymbols.

lines 2, 3

If the error did happen inside the procedure DefP, the application user is notified and P is reset to its default. The notification uses the original error description obtained using the intrinsic function errh::Message(err).

line 4

Each handled error will be marked as such. When an error handler finishes, it will delete the errors that have been marked as handled from the predeclared set errh::PendingErrors.

line 5

To discern the type of an error, errors are divided into categories. For each error, the category to which it belongs can be obtained using the function errh::Category(err). The error categories form a nested structure. For instance, both IO and Generation errors are Execution errors. The intrinsic function errh::InsideCategory(err) can be used to determine whether or not an error is within a particular category.

lines 6, 7

Translate the error by adapting information. In this example, only the message is actually adapted, but most parts of an error can be adapted. Note that in this else branch, the function errh::MarkAsHandled is not called, the result being that the adapted error message will appear in the messages/errors window.

line 8

In this branch, the error is not handled. An error that has not been handled when the error handler finishes will not be deleted. Instead, it is being displayed in the messages/errors window.

Local error handling by means of the OnError clause

The following template of a BLOCK statement illustrates local error handling by means of the OnError clause.

1    BLOCK
2       statement_1 ;
3       ...
4       statement_n ;
5    ONERROR err DO
6           ...
7           ...
8    ENDBLOCK ;


All errors occuring inside statement_1statement_n on lines 2 … 4 are handled by the error handler on lines 6 and 7, where err is an element parameter of the set errh::PendingErrors. Block statements can be nested, either directly in a single body, or in other procedures called from within block statements. This gives rise to a stack of error handlers as illustrated below. A detailed example of a local error handler is given in Runtime Libraries and the Model Edit Functions.

Error flow architecture

The global error handlers and the OnError error handlers are essential building blocks of the error handling framework of AIMMS. This error handling framework is illustrated in Fig. 1.

Construction of the error handler stack

At the start of each execution run, a new stack of error handlers is created. At the bottom of this stack is the standard handler To Global Collector. When the option Global_error_handler is set, the specified procedure is placed on top of this new stack. Additional handlers are placed on the stack by each OnError clause in a nested BLOCK statement.

Errors flowing through a handler stack

When raised, each error is set aside for handling by the topmost error handler. When the number of errors set aside reaches the limit specified by the option Errors_until_execution_interrupt, the execution is interrupted and resumes by executing the code in the topmost error handler. When the execution is not interrupted, but there are pending errors or warnings, the error handling code is executed after the completion of the last statement prior to the BLOCK statement.

Multiple errors may require handling

A single statement may result in multiple error messages, for instance a solve statement or a data assignment statement with several duplicate entries. Thus, even if the option Errors_until_execution_interrupt is 1 (its default), multiple errors may need to be handled. If multiple errors caused by a single statement are handled inside the OnError clause of a BLOCK statement, the code within the OnError clause will be executed unconditionally for every single error, unless you explicitly break away from theOnError clause.

Break away from handling

If you use a RETURN, HALT, BREAK or RAISE ERROR statement inside the OnError clause, the handling of any subsequent errors or warnings will be stopped. You are actually indicating that these further errors and warnings are no longer of interest and thus they will be automatically set as handled. A plain BREAK statement just breaks the error handling loop. If the Block statement is inside an outer loop statement like FOR or WHILE and you want to break from that loop, you need to use a loop string (see Advanced Use of WHILE and REPEAT).

SKIP in OnError

A plain Skip statement in the OnError clause simply skips the remaining statements and continues with the next error that needs to be handled. You can use a SKIP with a loop string to skip the statements of an outer loop statement. This will break away from the OnError clause as described above.

What to do with an error

For each error, the error handling code will decide whether to handle that error itself, let another handler handle the error, or ignore the error (as was already illustrated in the example above).

Handling an error inside a handler

Errors may also occur during the execution of the OnError clause or of a BLOCK statement or the global error handling procedure. These errors are handled by the next error handler in the stack of error handlers.

Error collector

When an error reaches the handler To Global Collector, it is sent to the Error and Warning Collector object which collects all errors that have fallen through the various handlers (if any). Errors in the Error and Warning Collector can be queried from within the AIMMS API or viewed from within the messages/errors window of the AIMMS GUI.

The predeclared module ErrorHandling

Errors to be handled can be queried using the following predeclared identifiers and intrinsic functions from the module ErrorHandling with prefix errh:

errh::PendingErrors

A predeclared set filled with the numbers of the errors that can be handled at this point.

errh::IndexPendingErrors

An index of the above predeclared set.

error parts

An error is made up of several parts; each of which can be obtained separately using the intrinsic functions below. Each of the functions below will raise an error of their own if err is not a valid error that can be handled at that point.

errh::Severity(err)

An element in errh::AllErrorSeverities is returned indicating the severity of the error.

errh::Message(err)

A string containing the error description is returned. This string is not empty.

errh::Category(err)

An element in errh::AllErrorCategories is returned indicating the category of the error.

errh::Code(err)

The element in errh::ErrorCodes that is returned by this function identifies the message code of the error. This element name may be cryptic; as it is primarily used for identification of the error within the AIMMS system.

errh::NumberOfLocations(err)

The number of locations relevant to this error. For compilation errors, there is typically only one relevant location. For an AIMMS initialization error there are no relevant locations. For an execution error the positions in all the active procedures are recorded. For an error during file read, at least the positions in the data file and the read statement are recorded. Similarly, for an error during the generation of a constraint, at least the constraint and the SOLVE statement are recorded as relevant positions.

errh::Node(err,loc)

An element in AllSymbols is returned for an error location inside the model. The optional argument loc defaults to 1 and should be in the range { 1 .. NumberOfLocations }. The element returned by this function is non-empty except for the first location when reading data from a file.

errh::Attribute(err,loc)

An element in AllAttributeNames.

errh::Line(err,loc)

An integer indicating the line number of the error in the attribute or file, or 0 if not known.

errh::Column(err)

An integer indicating the column position in an erroneous line being read from a data file. All errors when reading a data file are reported separately, such that the loc argument is not applicable.

errh::Filename(err)

A non-empty string is returned when reading from a data file. All errors when reading a data file are reported separately, and so the loc argument is not applicable.

errh::Multiplicity(err)

An integer indicating the number of occurrences of this error. Two errors are considered equal if they are equal in all of the following parts: Severity, Message, Category, Code and the first location (if available). The first location is the location in the file being read when the error occurs during a read statement, otherwise it is the statement being executed.

errh::CreationTime(err,fmt)

A string representing the creation time of the first occurrence of the error, formatted according to time format fmt.

errh::InsideCategory(err,cat)

Returns 1 if the error code of err falls inside the category cat.

errh::IsMarkedAsHandled(err)

Returns 1 if the error is marked as handled.

errh::Adapt(err, severity, message, category, code)

The error err is adapted with the components specified. Besides the mandatory argument err, there should be at least one other argument.

errh::MarkAsHandled(err,actually)

The error err is marked as handled if the argument actually is non-zero. Marked errors will not be passed to the next error handler. The default of the optional argument actually is 1. Using 0 will remove the mark from the error.

The log file aimms.err

AIMMS logs all errors and warnings to the file aimms.err as they are raised. The folder in which this file resides is controlled by the option Listing_and_temporary_files. The number of backups retained of this file is controlled by the option Number_of_log_file_backups.

## Raising Errors and Warnings

Raising errors

The RAISE statement is used to

• raise an error regarding a situation that cannot be handled, or to

• raise a warning regarding a situation that can be handled but might warrant further investigation.

The syntax of the RAISE statement is straightforward.

Syntax

raise-statement:

Example

In the following example an error is raised when the inflow of a node exceeds its capacity.

if inflow > stockCap then
RAISE ERROR "Inflow exceeds stock capacity" CODE 'TooMuchInflow' ;
endif ;


Error code and category

In order to enable an error handler to recognize the type of error being raised by a RAISE statement, that statement allows an optional error code to be specified. This is an element in the set errh::ErrorCodes. If the specified element does not yet exist, it is created and added to that set. The category of an error raised by the RAISE statement is fixed to 'User'.

Position information

AIMMS uses the line/procedure in which the RAISE statement is specified as the position information associated with the error. This permits the messages/errors window to open the attribute window of the procedure and place the cursor on the statement where the problematic situation is detected.

Raising warnings

Not only AIMMS itself but also procedures written in AIMMS may recognize situations that can be handled but might warrant closer inspection by the application user. For this purpose, the RAISE statement can raise a warning, for example:

if card( RawMaterialTraders ) = 0 then
RAISE WARNING "There are no raw material traders, this may lead to " +
"infeasibilities in the case of too many accepted deliveries." ;
endif ;


The handling of warnings generated by a RAISE statement is controlled by the option Warning_user, with default common_warning_default. The control of warning handling is further explained in Warnings.

## Legacy: Intrinsics with a Return Status

Legacy situation

AIMMS external procedures and intrinsic procedures can both return a status code indicating whether or not they were successful. A return value $$\leq 0.0$$ is interpreted as not successful, wheareas a return value $$> 0.0$$ is successful. In addition, when they are not successful, the error message is often left in CurrentErrorMessage, although this is only a guideline. The return value of a call to an intrinsic procedure is either

checked

As illustrated in the example:

retval := PageOpen(...) ;
if retval <= 0 then
... use CurrentErrorMessage ...
endif ;

not checked

As illustrated in the example:

PageOpen(...) ;


Available error handling methods

In the context of the error handling facility available in AIMMS, how should one handle the “checked” and “not checked” procedure calls when the return value is 0 and these procedures have not raised an error themselves? There are five error handling methods available to choose from:

ignore

An error is never raised for an error occurring inside such a procedure, whether or not the return status is checked.

raise_warning_when_not_checked

A warning is only raised if the return status of an intrinsic procedure is not checked.

raise_when_not_checked

An error is only raised if the return status of an intrinsic procedure is not checked.

raise_always_warning

A warning is raised whether or not the return status is checked.

raise_always

An error is raised whether or not the return status is checked.

Which choice of error handling method is best depends on the application and can be controlled using the options:

Intrinsic_procedure_error_handling

for procedures with a return status supplied by AIMMS and

External_procedure_error_handling

for externally supplied procedures.

The values of these options are the names of the error handling methods described above. The default of both these options is raise_when_not_checked. For projects created prior to the introduction of the error handling facilities in AIMMS (i.e. created in AIMMS 3.9 or lower), these options generate the non-default value raise_warning_when_not_checked in order to notify the model developer but do not change the existing behavior of such projects significantly.

## Warnings

Warnings

AIMMS recognizes and warns about several types of possibly problematic situations. These situations might warrant further investigation. As with most other languages, AIMMS warns against the use of identifiers before initializing them. But unlike other languages, AIMMS also warns against the inconsistent use of units of measurement (such as a comparison of a volume against a weight), or of model formulations for which AIMMS can detect either compiletime or runtime issues that lead to sub-optimal performance or ambiguous results. A selection of performance-related warnings is discussed in Using AIMMS’ Advanced Diagnostics.

Complete flexibility

The desired handling of each of these situations depends on the developer and the application; varying from treating it as an error to fully ignoring it. To permit complete flexibility, there is separate option to control the reporting of each type of problematic situation recognized.

Grouping Warning options

Although all warnings can be controlled individually, this is not the most convenient way to employ the diagnostics provided by these warnings. When entertaining a new idea (quick prototyping), most modelers understandably do not want to be bothered by various warnings and want to be able to turn them all off. To facilitate this, all the warnings have been grouped into either common or strict warnings, and the associated options assume default value for common and strict warnings. Thus, all diagnostic warnings can be switched off by just changing the options that control these defaults. For normal development work it is advisable to at least turn the common warnings on. In addition, we would encourage to turn on the strict warnings during application tests.

Choosing the option setting

In order to implement the above scheme and still permit full flexibility, each option controlling the detection of a type of problematic situation can take on one of the following values:

error

The situation is marked as an error and treated as an error.

warning_handle

The warning is raised in the current error handler, but does not count toward the interruption of normal execution.

common_warning_default

The value of the option Common_warning_default is used.

warning_collect

The warning is raised in the Global_error_collector, bypassing the stack of error handlers.

strict_warning_default

The value of the option Strict_warning_default is used.

off

The warning is ignored.

The default of these options is either common_warning_default or strict_warning_default, thereby effectively dividing these options into common and strict groups. The range of options for common_warning_default and strict_warning_default is {off, warning_collect, warning_handle, error}. The default of the option common_warning_default is warning_handle and the default of the option strict_warning_default is off.

## Train on error handling

You may refer to our online training about error handling following this link to apply error handling on concrete models.