Debugging in MQL

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MQL is a really basic scripting language. Fixing bugs in MQL, unfortunately, is not simple at all. The MetaEditor compiler that MetaQuotes supplies simply does not include the advanced tools that most programmers are used to using.

MQL debugging problems

Visual Studio and other sophisticated IDEs (integrated development environments) incorporate a number of features that make it clear to fix code while the programmer writes it. The biggest illustration of this are break points. A break point is a point in the program where the compiler informs the computer to cease running the code when it arrives at that stipulated line.

Take the example where a trailing stop implements a new stop incorrectly. The normal instinct for most programmers would be to run the expert advisor on the visual backtester, then introduce break points on the lines of the program immediately after the trailing stop calculations. Break points stop the code, allowing the programmer to peer inside the brains of the EA to see what it thought at the time it made a decision. The key advantage in Visual Studio is that the values of all of the variables are clearly visible. It is possible to walk through the program step by step. Whenever one of the steps does not follow the preferred rules, the necessary change is usually obvious. MetaQuotes thankfully included break points in MQL5. They are not available in MQL4.

The lack of full intellisense support inhibits my programming speed more than anything. Intellisense notices the use of reserved words like OrderSelect() or ObjectGet(). The MetaEditor includes an immature intellisense, but it lacks the fine details that make it so convenient in Visual Studio.

I am accustomed to programming in C# where I can type the first few letters of a variable or class, then the IDE fills out the rest. When I type “Mes” in C# and push the space bar, I know that the MessageBox option will show up (assuming that I assigned the required namespace). The MetaEditor includes a list of candidates for reserved words. The programmer must then either pick the option with the mouse or press enter.

I know it seems trivial to require pushing enter in lieu of the space bar, but think about how many times code resuses the same reserved words or variables. The extra presses of keys really do add up to a lot of unnecessary typing actions. That’s doubly true for a thirty year old that already wears a wrist brace for carpal tunnel pain.

The MetaEditor’s biggest weakness is that it does not study variable names. We often compose EAs that comprise several thousand lines of code. Following the names of tens of variables poses its own challenges. When the coder types in the same set of variable names repeatedly, it would be nice to simply type the first three letters and move on. Copy and paste might provide a decent alternative. The problem is that variables usually group together. You cannot keep 5 different copy and paste items readily available.

The MetaEditor allows functions to return invalid types. Functions declared as double can return strings, integers or nothing at all. The MQL4 compiler does not track whether or not these are valid. It leaves it up to the programmer to discover the invalid type during real time testing. This oversight is a nightmare for the unwitting programmer that mistakenly returns the wrong type.

This is even more true when a double function is erroneously returned to an integer variable. MQL4 does not prevent illegal double to int casts. Even worse, the expert advisor continues running with a 0 value for the interger instead of throwing an exception or error message. I cannot count how many hours that I’ve wasted tracking down variables that look dead-on, only to realize that I declared the wrong data type. This usually happens when I’m on autopilot, pounding out code. What appears efficient at the time frequently costs several hours of hair pulling irritation.

MQL troubleshooting techniques

The MQL programmers on staff here usually resort to any of the following techniques. You may find that using them in groups aids with improving the troubleshoot process even more.

Debug compiler error

This one can be the most frustrating. The MetaEditor attempts to hint at which line of code causes the compiling error. I say attempts because it gets it wrong more often than it gets it right. There’s nothing more irritating than looking at a perfectly legitimate line of code that the compiler flags as problematic.

I almost always resort to commenting out more and more large blocks of the expert advisor until the issue goes away. I commence with commenting out a single line of code. If that doesn’t work, then I comment out ten lines. If that doesn’t work, I might comment out entire functions or blocks of code. When the compiler finally runs properly, you know that the last section of commented out code incorporates the bug.

Next, you backtrack. Commence with making the offending commented-out section smaller and smaller until the error reappears. Now, you have finally zeroed in on the actual source of the problem.

Troubleshoot in real time or on the backtester

My preferred manner of debugging is to comment most of the relevant decision information onto the screen, which is done using the Comment() function. I then run the visual backtester, watching how the data behaves in relation to the visual information.

On screen comments are essentially jury-rigged break points. Controlling how and when they appear allows the coder to step through the code to uncover the issue. The only difference is that comments do not forcefully prevent the code from running. The text which appears is very small. Aside from that, I really like that fact that it’s so robust. The comment function always works without a hitch, making it the best friend of a coder that’s troubleshooting code.

Taking screenshots takes this to the next level. Whenever clients ask questions about why an ea behaved a certain way, the easiest answers come with screenshots. Commenting the imitation break points usually provide bullet proof answers – the coder and consumer can literally see what the Expert advisor thought at the time it made a decision. MQL4 offers the WindowScreenShot() function to do this.

The EAs that we program always take screenshots during pivotal actions, such as placing a trade or adjusting an exit condition. The goal is to provide a visual record of every decision with an eye to answering future inquiries about the action.

Our default template includes a true/false variable called WriteScreenshots. Traders control whether they want to bother with this debugging feature or not. The only downside to it is that every recorded action eats up about 50kb of hard drive space.

Log files represent the last bug fixing option. The major drawback is that they are so ugly and arduous to read. I almost always prefer screenshots for this reason.

Nonetheless, log files do have their place. The main way to use them is as error catchers. Whenever a process goes awry due to an issue with either MetaTrader itself or with a broker transaction, the log file is the easiest place to record it.

Bug fixing MQL files is a talent that takes time to learn. The methods at the coder’s disposal are very different from those available to higher level languages. Once you get accustomed to functioning with the much simpler tools in the MetaEditor and MetaTrader, the bug fixing process goes a lot quicker.

Source by Shaun Michael Overton

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