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Error trapping with Dyalog APL: Difference between revisions
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Error trapping with Dyalog APL (view source)
Revision as of 22:09, 10 September 2022
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It is good programming style to avoid using numbers in code. Instead of talking about 1001, for example, we should use a meaningful name: | It is good programming style to avoid using numbers in code. Instead of talking about 1001, for example, we should use a meaningful name: | ||
< | <syntaxhighlight lang=apl> | ||
⎕CS 'Events' #.⎕NS '' | ⎕CS 'Events' #.⎕NS '' | ||
⎕FX 'r←StopVector' 'r←1001' | ⎕FX 'r←StopVector' 'r←1001' | ||
| Line 33: | Line 33: | ||
We need also a user-defined event for restarting the application. This is explained soon: | We need also a user-defined event for restarting the application. This is explained soon: | ||
< | <syntaxhighlight lang=apl> | ||
#.Events.RestartAppl←501 | #.Events.RestartAppl←501 | ||
</syntaxhighlight> | </syntaxhighlight> | ||
| Line 39: | Line 39: | ||
According to the help file, users should use the range from 500 to 999 to define their own events. | According to the help file, users should use the range from 500 to 999 to define their own events. | ||
== Setting < | == Setting <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> == | ||
< | <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> allows us to implement a general mechanism on a global level. For discussion purposes let's assume the following: | ||
# < | # <syntaxhighlight lang=apl inline>⎕LX</syntaxhighlight> is set to run function <syntaxhighlight lang=apl inline>Run</syntaxhighlight> | ||
# This function calls 3 sub-functions: < | # This function calls 3 sub-functions: <syntaxhighlight lang=apl inline>Initial</syntaxhighlight>, <syntaxhighlight lang=apl inline>Work</syntaxhighlight> and <syntaxhighlight lang=apl inline>Shutdown</syntaxhighlight> | ||
# < | # <syntaxhighlight lang=apl inline>Initial</syntaxhighlight> initializes the application: it opens files, interprets an INI file, takes the Windows registry into account, builds the GUI and so forth. | ||
# < | # <syntaxhighlight lang=apl inline>Work</syntaxhighlight> simply runs <syntaxhighlight lang=apl inline>⎕DQ</syntaxhighlight> | ||
# < | # <syntaxhighlight lang=apl inline>Shutdown</syntaxhighlight> cleans up: it closes files, says good-by. | ||
=== Solving the stop vector problem === | === Solving the stop vector problem === | ||
Let's start with solving the stop vector problem: | Let's start with solving the stop vector problem: | ||
< | <syntaxhighlight lang=apl> | ||
⎕TRAP←⊂(#.Events.StopVector 'E' '→⎕LC') | ⎕TRAP←⊂(#.Events.StopVector 'E' '→⎕LC') | ||
</syntaxhighlight> | </syntaxhighlight> | ||
< | <syntaxhighlight lang=apl inline>#.Events.StopVector</syntaxhighlight> returns 1001 which is the event number a stop vector is associated with. As soon as APL stops on a stop vector <syntaxhighlight lang=apl inline>⎕EN</syntaxhighlight> is set to 1001. This event can be caught with <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight>, so we can tell APL to execute (<syntaxhighlight lang=apl inline>'E'</syntaxhighlight>) the expression given as third argument. In this case it tells APL to simply ignore stop vectors by resuming execution. | ||
=== Preventing users from interrupting an application === | === Preventing users from interrupting an application === | ||
| Line 61: | Line 61: | ||
Here, however, we will use this simple approach: | Here, however, we will use this simple approach: | ||
< | <syntaxhighlight lang=apl> | ||
events←#.Events.WeakInterrupt #.Events.StrongInterrupt | events←#.Events.WeakInterrupt #.Events.StrongInterrupt | ||
⎕TRAP,←⊂(events 'E' '→⎕LC') | ⎕TRAP,←⊂(events 'E' '→⎕LC') | ||
| Line 68: | Line 68: | ||
=== Restarting the application === | === Restarting the application === | ||
For reasons explained in a minute we now have to define the “Restart the application” procedure. For this, for the first time we do not use the < | For reasons explained in a minute we now have to define the “Restart the application” procedure. For this, for the first time we do not use the <syntaxhighlight lang=apl inline>'E'</syntaxhighlight> statement but the <syntaxhighlight lang=apl inline>'C'</syntaxhighlight> statement. The ''C'' is short for ''Cut back''. This instructs APL to cut the status indicator back to the level where <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> '''is localized''' – that is not necessarily where it was set – and execute the expression in the 3rd argument there. However, if <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> is not localized at all, i.e. it is in the workspace, the status indicator is cut back completely and the expression is executed in the workspace. | ||
< | <syntaxhighlight lang=apl> | ||
⎕TRAP,←⊂(#.Events.RestartAppl 'C' '→∆Restart') | ⎕TRAP,←⊂(#.Events.RestartAppl 'C' '→∆Restart') | ||
</syntaxhighlight> | </syntaxhighlight> | ||
To make this work the function in which < | To make this work the function in which <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> is localized must have a label <syntaxhighlight lang=apl inline>∆Restart</syntaxhighlight> or a function that returns a valid line number to branch to of course. | ||
=== Catching Errors === | === Catching Errors === | ||
If an unexpected error occurs, we want to execute a particular function to do the hard work. | If an unexpected error occurs, we want to execute a particular function to do the hard work. | ||
< | <syntaxhighlight lang=apl> | ||
⎕TRAP,←⊂((0 1000) 'E' '#.HandleError' ) | ⎕TRAP,←⊂((0 1000) 'E' '#.HandleError' ) | ||
</syntaxhighlight> | </syntaxhighlight> | ||
The 0 stands for all the events from 1 to 999 while the 1000 stands for all events larger than 1000. | The 0 stands for all the events from 1 to 999 while the 1000 stands for all events larger than 1000. | ||
< | <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> may contain more than one error catching group. Since the contents of <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> is scanned from left to right, a statement will ONLY be executed for an event not processed earlier. That is the reason why we must define the restart event first. | ||
For example, in the following statement: | For example, in the following statement: | ||
< | <syntaxhighlight lang=apl> | ||
⎕TRAP←(333 'E' 'expA') (0 'E' 'expB') | ⎕TRAP←(333 'E' 'expA') (0 'E' 'expB') | ||
</syntaxhighlight> | </syntaxhighlight> | ||
event 333 will be caught by the 1st group and NOT by the 2nd even though 0 stands for “events from 1 to 999”. Only the expression < | event 333 will be caught by the 1st group and NOT by the 2nd even though 0 stands for “events from 1 to 999”. Only the expression <syntaxhighlight lang=apl inline>expA</syntaxhighlight> will be executed. | ||
== The < | == The <syntaxhighlight lang=apl inline>#.HandleError</syntaxhighlight> function == | ||
The < | The <syntaxhighlight lang=apl inline>#.HandleError</syntaxhighlight> function should do at least the following: | ||
* Perhaps neutralize < | * Perhaps neutralize <syntaxhighlight lang=apl inline>⎕LX</syntaxhighlight> | ||
* Save the < | * Save the <syntaxhighlight lang=apl inline>⎕DM</syntaxhighlight> and <syntaxhighlight lang=apl inline>⎕EN</syntaxhighlight> settings | ||
* Save the snapshot | * Save the snapshot | ||
* Maybe create an HTML page with general information about the error | * Maybe create an HTML page with general information about the error | ||
| Line 110: | Line 110: | ||
Keep in mind that you want to have an easy opportunity to test the system with error trapping. So you may need another parameter that tells the system that error trapping has to be used. Last but not least, there should be an easy opportunity to let the application crash on purpose. I prefer to have a “developers menu”, which is displayed only to developers. Among other useful commands it offers a “Let's crash” option. | Keep in mind that you want to have an easy opportunity to test the system with error trapping. So you may need another parameter that tells the system that error trapping has to be used. Last but not least, there should be an easy opportunity to let the application crash on purpose. I prefer to have a “developers menu”, which is displayed only to developers. Among other useful commands it offers a “Let's crash” option. | ||
=== Control Structure < | === Control Structure <syntaxhighlight lang=apl inline>:Trap</syntaxhighlight> === | ||
If you use < | If you use <syntaxhighlight lang=apl inline>:Trap</syntaxhighlight>, keep in mind that <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> and <syntaxhighlight lang=apl inline>:Trap</syntaxhighlight> are both taken into account. That means that in case of | ||
< | <syntaxhighlight lang=apl> | ||
:Trap 0 | :Trap 0 | ||
-'a' | -'a' | ||
| Line 121: | Line 121: | ||
</syntaxhighlight> | </syntaxhighlight> | ||
the error caused by the < | the error caused by the <syntaxhighlight lang=apl inline>-'a'</syntaxhighlight> statement is caught by the <syntaxhighlight lang=apl inline>:Else</syntaxhighlight>, while the <syntaxhighlight lang=apl inline>.</syntaxhighlight> is caught by the <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> setting. | ||
When using < | When using <syntaxhighlight lang=apl inline>:Trap</syntaxhighlight> try to be as specific as possible. For example, this code is faulty: | ||
< | <syntaxhighlight lang=apl> | ||
:Trap 0 | :Trap 0 | ||
filename ⎕FTIE 0 | filename ⎕FTIE 0 | ||
| Line 132: | Line 132: | ||
</syntaxhighlight> | </syntaxhighlight> | ||
because it tries to create a file not only if this file does not already exist but also if the current user lacks the right to tie it, for example because somebody else has already tied it exclusively. Therefore, it is a better to be specific: | because it tries to create a file not only if this file does not already exist but also if the current user lacks the right to tie it, for example because somebody else has already tied it exclusively. Therefore, it is a better to be specific: | ||
< | <syntaxhighlight lang=apl> | ||
:Trap 22 | :Trap 22 | ||
filename ⎕FTIE 0 | filename ⎕FTIE 0 | ||
| Line 139: | Line 139: | ||
:EndTrap | :EndTrap | ||
</syntaxhighlight> | </syntaxhighlight> | ||
The best idea, however, is to use < | The best idea, however, is to use <syntaxhighlight lang=apl inline>⎕NEXISTS</syntaxhighlight> to check the file for already being created. In general it is a good idea to use error trapping only for extraordinary problems. | ||
=== The < | === The <syntaxhighlight lang=apl inline>⎕SIGNAL</syntaxhighlight> system function === | ||
Note that an event which is < | Note that an event which is <syntaxhighlight lang=apl inline>⎕SIGNAL</syntaxhighlight>led can be intercepted with <syntaxhighlight lang=apl inline>⎕TRAP</syntaxhighlight> but not <syntaxhighlight lang=apl inline>:Trap</syntaxhighlight> | ||
If you execute this function: | If you execute this function: | ||
< | <syntaxhighlight lang=apl> | ||
∇test | ∇test | ||
⎕TRAP←501 'E' '⎕←''caught by ⎕TRAP''' | ⎕TRAP←501 'E' '⎕←''caught by ⎕TRAP''' | ||
| Line 156: | Line 156: | ||
</syntaxhighlight> | </syntaxhighlight> | ||
you get this: | you get this: | ||
< | <syntaxhighlight lang=apl> | ||
caught by ⎕TRAP | caught by ⎕TRAP | ||
</syntaxhighlight> | </syntaxhighlight> | ||
| Line 163: | Line 163: | ||
A very easy way to create problems in the future is to do this: | A very easy way to create problems in the future is to do this: | ||
< | <syntaxhighlight lang=apl> | ||
:Trap 0 | :Trap 0 | ||
DoSomethingHere | DoSomethingHere | ||
| Line 174: | Line 174: | ||
When you use error trapping, make sure that you can switch off error trapping on a general level. The easiest way to implement this idea is something like this: | When you use error trapping, make sure that you can switch off error trapping on a general level. The easiest way to implement this idea is something like this: | ||
< | <syntaxhighlight lang=apl> | ||
:Trap #.ErrorTrapFlag/0 | :Trap #.ErrorTrapFlag/0 | ||
DoSomething | DoSomething | ||
| Line 181: | Line 181: | ||
EndTrap | EndTrap | ||
</syntaxhighlight> | </syntaxhighlight> | ||
If the flag is true, error trapping is active, if not, the < | If the flag is true, error trapping is active, if not, the <syntaxhighlight lang=apl inline>DoSomeThing</syntaxhighlight> statement will fail if an error occurs. This makes is much easier to debug an application. | ||
You might need a more sophisticated mechanism for this, because under some circumstances you want to switch off most but not all error trapping statements. For example, if you use a logging mechanism which is logging every user action for analyzing purposes, the code doing this may cause an interrupt itself, for example because the disk is full which holds the logging files. In such a case it might be inappropriate that the logging code breaks the application. Therefore, you might control this code with < | You might need a more sophisticated mechanism for this, because under some circumstances you want to switch off most but not all error trapping statements. For example, if you use a logging mechanism which is logging every user action for analyzing purposes, the code doing this may cause an interrupt itself, for example because the disk is full which holds the logging files. In such a case it might be inappropriate that the logging code breaks the application. Therefore, you might control this code with <syntaxhighlight lang=apl inline>:Trap</syntaxhighlight>-statements. | ||
In such a case it might be a good idea to control the behavior of the application on different levels for code which is really essential in terms of business logic, for example, and for code which is not essential. | In such a case it might be a good idea to control the behavior of the application on different levels for code which is really essential in terms of business logic, for example, and for code which is not essential. | ||
| Line 191: | Line 191: | ||
== Code == | == Code == | ||
{{Collapse|The below workspace contains all the code needed to implement the ideas discussed above.|< | {{Collapse|The below workspace contains all the code needed to implement the ideas discussed above.|<syntaxhighlight lang=apl> | ||
(⎕IO ⎕ML ⎕WX)←1 1 3 | (⎕IO ⎕ML ⎕WX)←1 1 3 | ||