# Driving with D

Here is what comes to mind when I think of D: fast, expressive, easy, and… driving? That’s right, I drive with D.

Enter my venerable Holden VZ Ute daily driver. From the factory, it came with a rubbish four-speed automatic gearbox. During 18 months of ownership, I destroyed four gearboxes. I could not afford a new vehicle at the time (I’m a 20-year-old Australian computer science student at Monash University), so I had to get creative. I purchased a rock-solid, bulletproof, six-speed automatic gearbox from another car. But that’s where the solutions ended. To make it work, I had to build my own circuit board, computer system, and firmware to control the solenoids, hydraulics, and clutches inside the gearbox, handle user input, perform shifting decisions, and interface to my car by pretending to be the four-speed automatic.

I’m quite proud of my solution. It can perform a shift in 250 milliseconds, which is great for racing. It has a steep first gear, giving it a swift takeoff. It has given some more powerful cars a run for their money. It’s got flappy paddles, diagnostic data on the screen, and the ability to go ahead and change the way it works whenever I want.

Here’s a very old video of the system working. It’s not representative of the current system—that ghastly blue screen is gone, the speedo works, and shifting has improved.

The computer is split into two parts: the user interface board, which drives an OLED display and uses an STM32F042, and the mainboard, which handles everything else, utilizing an STM32F407. The two cooperate over a CAN bus (Controller Area Network). All the firmware to handle this is written in D.

I picked D (as -betterC) because of its ingenious Uniform Function Call Syntax (UFCS), design by contract, metaprogramming, ease of interfacing with C, unit testing, portability, shared, @safe, and codebase maintenance features. Another bonus is the helpful, welcoming community. It has genuinely been a joy discussing D on the forums, and with the founders and community leaders.

Uniform Function Call Syntax (UFCS)
This has made my code significantly clearer. My code can accurately follow the flow of data without polluting my stack with single-use variables, nesting many function calls, or other sorts of clutter.
Here is an example of how you could potentially use it in an ECU (Engine Control Unit):

immutable injectorTime = airStoich(100.kpa, 25.degCelsius)
.airMass
.fuelMass((14.7f).afr)
.fuelMol
.calculateInjectorWidth;

This is equivalent to:

immutable injectorTime =
calculateInjectorWidth(
fuelMol(
fuelMass(
airMass(
airStoich(kpa(100), degCelsius(25))
),
afr(14.7)
)
)
); // brackets have been expanded for reading clarity

Please note: the values in this example are hardcoded to simplify the code and demonstrate how UFCS can give a unit of measurement to a value.

Both representations are valid D code; you can use either.

With UFCS, there’s no need to read the code backward or count your brackets, no need to use a gazillion single-use variables. Function calls mirror the flow of data. It’s concise.

Design by Contract
D’s contract programming is quite similar to Ada’s. Functions can be marked with preconditions, designated by in, and postconditions, designated by out. Should a contract fail, an assertion is thrown.

// This demonstrates D’s contract programming for a function.
// It uses the short-hand expression based syntax.
int iHaveAContract(void* ptr)
in(ptr !is null) // this is a precondition, if ptr is null, an assertion is raised
in(ptr !is null, "ptr is null :(") // this is a precondition, if ptr is null, an assertion with the error message "ptr is null :(" is raised
out(result; result > 0) // this is a post condition, it captures the return value as result and tests it
out(result; result > 0, "result was too low") // this is a post condition, it captures the return value as result, tests it, and if it fails, raise an assertion with the message "result was too low"
{
// normal function code here
}

If you want to do something a bit more complex in your contracts, an alternative syntax is available:

int iHaveAContract(void* ptr)
in {
assert(ptr !is null); // this is equivalent to in(ptr !is null) from above.
MyStruct* ms = cast(MyStruct*)ptr; // we can introduce variables local to this contract
assert(ms.blah == 2, "MyStruct.blah must be 2!"); // test ms.blah, if it fails, raise an assert with an error message
}
out(result) { // captures the return value as result
int squareIt = result * result;
assert(squareIt == 4);
}
do { // this designates the function body
// normal function code here
}

void main(string[] args)
{
auto i = iHaveAContract(null); // this will violate the contract
}

Structs and classes can include contracts, called invariants, that sanity check the state of an instance for its whole lifetime. Invariants are checked after the constructor is run, before the destructor is run, and before and after public function calls.

struct MyStruct
{
int blah;

invariant {
assert(blah == 2, "blah must always be 2 for some reason!");
}
// shorthand:
invariant(blah == 2, "blah must be 2 for some reason!");

// The invariant is checked at function entry and exit. If value is anything other than 2, the invariant will fail when the function exits
void setBlah(int value)
{
blah = value;
}
}

void main(string[] args)
{
MyStruct s; // invariant is run after construction
s.setBlah(3); // invariant contract will be violated.
}

Metaprogramming
The Don’t Repeat Yourself (DRY) principle is often touted by programmers. D’s metaprogramming is an incredible tool to achieve that goal. I use it in my CAN bus implementation. For example:

struct CANPacket(ushort ID) {
enum id = ID;
ubyte[8] data;
}
alias HeartbeatPacket = CANPacket!10;
alias BeepHornPacket = CANPacket!140;

I’ve got specific aliased types as HeartbeatPacket and BeepHornPacket, but I haven’t needed to repeat any code. They all follow the same underlying structure, so if I modify CANPacket, every alias is also updated.

Maybe you want a more descriptive CAN packet? Mixin templates can help with that!

struct GenericCanPacket
{
ushort id;
ubyte[8] data; // 8 bytes to store the CAN packet payload
}

struct HeartbeatPacket
{
ubyte deviceID; // first byte is the device ID
ubyte statusID; // second byte is the status
}

To translate a GenericCanPacket to the descriptive HeartbeatPacket, we can use mixin templates.

mixin template CanPacketHelperFunctions(ushort ID)
{
enum id = ID;

// typeof(this) means that the return type of readFromGeneric will be the struct this template is instantiated in.
static typeof(this) readFromGeneric(const ref GenericCanPacket p)
in(p.id == ID, "Generic packet cannot be converted!")
{
// do some cast
}
}

struct HeartbeatPacket
{
// the stuff declared in CanPacketHelperFunctions is pretty much copy-pasted (not literally) into here
mixin CanPacketHelperFunctions!10;

ubyte deviceID; // first byte is the device ID
ubyte statusID; // second byte is the status
}

void main()
{
GenericCanPacket generic;
generic.id = 10;
generic.data = [ 2 /* deviceID */, 3 /* statusID */ ];

writeln(HeartbeatPacket.id); // the packet ID is 10
writeln(heartbeat.deviceID); // 2
writeln(heratbeat.statusID); // 3
}

The mixin template CanPacketHelperFunctions can be used over and over for all sorts of packet representations, and since it is only declared once, the implementation remains consistent across all types that use it.

Interfacing to C
I frequently must communicate with my microcontroller’s HAL and RTOS; D’s C interface made that a breeze. Just add an extern(C) and it’s good to go.

extern(C) c_setPwm(int solenoid, void* userData); // declaration
c_setPwm(4, null); // usage, pretty easy!

Unit Testing
D’s built-in unit testing has saved me from blowing my foot off a few times. I can run all my unit tests on Windows to guarantee logical correctness, and then build a final target for my microcontroller. Here is an example:

struct MyStruct
{
int x;
int squareIt() { return x * x; }
}

unittest
{
MyStruct s;
s.x = 9;
assert(s.squareIt == 9 * 9); // If for some reason the implementation breaks, then the unit test fails
}

Deprecation
Codebases can be ever-changing, and sometimes certain functionality may no longer be considered good practice but must be retained for legacy reasons. D provides a way to explicitly mark this in code. Any use of such deprecated code will trigger a deprecation warning from the compiler. Example:

deprecated struct Example
{
// ...
}

// This deprecation includes a reason
deprecated("This is the reason for deprecation..") struct ExampleWithMessage
{
// ...
}

void main()
{
// This will generate the following compiler warning:
// "Deprecation: struct Example is deprecated"
Example e;

// This will generate the following compiler warning:
// "Deprecation: struct ExampleWithMessage is deprecated - This is the reason for deprecation.."
ExampleWithMessage ewm;
}

Of course, deprecated can be applied to all sorts of things, not just structs.

Portability
Following on from above, D supports a surprisingly large number of targets via GDC and LDC. If it weren’t for D’s portability, I would have had to write my project in C++ (ugh). I use LDC, and cross-compiling can be performed by simply adjusting my command line arguments.

Shared
shared is D’s way of guarding against multi-threaded access of code. It’s not perfect, but I use it as-is, and I think it works well. I do have multiple threads in my codebase, and they need to synchronize data. I mark certain variables as shared, which means I must take special care accessing that data. It works with system locks and mutexes. While locked, I can cast shared away and use it like a normal variable. This is handy with structs and classes.

shared int sensorValue;
sensorValue = 4; // using it like a single-thread variable, error
atomicStore(sensorValue, 4); // works with atomics

SafeD
@safe exists to prohibit sketchy memory activities and enforce best behavior. I haven’t had to fight @safe much yet because I don’t do anything wicked with my memory, but it is comfortable knowing that if I am going to make a mistake, the compiler can assist me in stopping it.

Mental Friction
Adam D. Ruppe puts it succinctly: D has low mental friction. The flexibility and expressiveness of the language make it easy to translate one’s thoughts into written code and maintain productivity. I don’t have to fight D much. This is my personal opinion, but I feel like D is the language in which I’m most productive.

### Final Thoughts

D is the perfect fit for this sort of project— I think it’s going to have a bright future ahead in the embedded world. I’m going to continue using D for my projects. I’ve got another D-powered automotive project in the works which I hope to show off in the future. Even if D isn’t yet suitable for your project, keep an eye on it. D has been making enormous strides in the past few years, especially in regards to memory safety.

The examples shown in this article are purely meant to demonstrate how D’s features can be used in the real world. Do not take them as gospel as to how you should program.

# D-lighted, I’m Sure

For me, finding D is the most recent step along a road winding back at least a dozen years. I’d been searching for a cross-platform development language/environment (POSIX and Windows, but not so much mobile since my search began before mobile was really a thing) and at this point, D fits better than anything else I’ve tried. I won’t go out on a limb and say it’s the Holy Grail of X-Plat, but at the very least, it’s brought some fun back into coding for me. And whenever I massage a hunk of code until it finally works… well, I’m addicted to those little victories.

The first language I tackled back in the mid-oughts to meet this end was PHP. I’d been a web developer for a few years when I found out that PHP had a standalone desktop version. When I then stumbled across Andrei Zmievski’s PHP-GTK, lightbulbs went on and fireworks went off. For a while. The big drawback I found with PHP-GTK was that no PHP compiler could handle the GTK end. So after a few years of patiently hoping someone would tackle this while I wrote nearly 40 blog posts on its use, I started looking elsewhere.

Back in the 1990’s, I’d been steeped in Javascript and HTML, writing simple online apps for banking clients out of Vancouver, BC and later, Bancroft, Ontario. With such a background, when I ran across Electron a few years ago, it seemed like a good fit. I assumed learning it would be easy-peasy-lemon-squeezy, but a lot about the nature of web development languages had changed. To boot, several more languages, standards, and paradigms had been thrown into the mix, so to say the least, I found it all confusing and more than a little intimidating. What I really wanted was one language, an easy distribution system, and a GUI toolkit that didn’t necessitate balancing style sheets with front- and backend code as well as JSON files. And Electron, unfortunately, needs to drag Chromium along for the ride in a little metaphorical red wagon. It’s a solution, but not the one I was looking for.

Then last year, I stumbled onto D. I’d been hearing about it for years, but I hadn’t read much about it. I didn’t realize it was available across so many POSIX platforms. I also didn’t realize it embraced the OOP paradigm and so hadn’t given it much thought. I liked the subtle humor of D being next in line after B, C and A (which oddly enough came along later than B and C), but after a brief smile, I paid no more attention until last October.

When I finally took a good look, I realized that with its curly braces and OOP propensity, D runs right up my street. But before rolling up my sleeves, I made sure there was a GUI toolkit I could use, something that didn’t necessitate balancing three differing paradigms at once. I found a list of GUI toolkits on the D Wiki and was gratified to see GtkD among them. So for the last two plus months, I’ve been putting most of my effort into learning how D and GtkD work together.

## Perspiration and Refreshments

It may or may not be important to know this, but I don’t have a CS degree. I’m completely self-taught, a process that started while stuck in the middle of a frozen nowhere for three weeks more than 30 years ago. But that’s a whole other story. My point is, there are holes in my education. That’s what happens when you follow your nose instead of a syllabus.

Because of that, some of the intricacies of D elude me and may always do so. Although I’ve read Ali Çehreli’s chapter on the subject (from Programming in D) I have no idea what mixins are or why I’d want to use them. And templates seem like a good idea, but I don’t know why. I blame my lack of formal CS education for this, but I’m quite comfortable with classes and objects, so I’m not sweating it.

I was first introduced to OOP and the Gang of Four when I was learning PHP, so D covers familiar territory in that regard. Curly braces are another thing I feel quite at home with, having used them for most of my programming life.

But I’m finding the familiarity of D to be a bit of a stumbling block as well. It’s just different enough from C and PHP to mean I have to work hard at pounding those differences into my brain. I deal with it through rote typing of example code. I figure if I copy out enough D code instead of lazing along with copy-n-paste, eventually I’ll push C and PHP far enough into the background that I can see past them. And I keep Mr. Çehreli’s book handy so I don’t go completely off the rails. It’s been quite helpful. And speaking of helpful…

## The Ecology of D

### Forums

So far, I’ve signed up for two D-oriented forums, one at dlang.org and the other at gtkd.org. I have yet to find an unfriendly avatar. Everyone I’ve encountered seems willing to jump in and help. To contrast these forums with one I’ve been active on for a few years (not mentioning names, but this other forum is related to art and graphics, not programming) on the D forums I haven’t been insulted, nor have I been questioned for looking at things from a (warning: film term) Dutch angle—which is one of the things I do best. That comes from my art background, I suspect. I was quite the rabble rouser in art college. (Just ask Alan Wood the installation artist about our knock-down-drag-out shouting match in the cafeteria of Emily Carr College if you don’t believe me. But again, that’s a whole other story.)

On dlang.org, I mostly hang in the Learn sub-forum, which I suppose is only natural at this stage. It’s probably the most polite bunch I’ve run across on a forum ever, and I’ve been frequenting forums since the hoary days of the BBS when 1200 BAUD was lauded as the fastest thing since the 427 hemi.

Mike Wey seems quite patient for someone who answers more-or-less the same question over and over again on the GtkD forum. The only negative I found with that forum was technical. I signed up, made some posts, and when I went back to sign in a second time, I had to reregister. But I was still identified as the same Ron Tarrant who signed up the first time (I think) so perhaps that’s how things are supposed to be. It’s unusual, but it works.

I will also mention that the GtkD documentation pages are a mind-bender, but because this is where I’m planning to spend a lot of time over the next while, I’ve decided to pitch in and help make things more accessible.

I’m drawing on experiences writing my PHP-GTK blog back in 2006 and porting a bunch of code examples and tutorials into GtkD. I went so far as to buy a domain name (gtkDcoding.com) in preparation for launching a site and blog covering all this. That’s how I deal with learning curves and have done since I wrote that series of BASIC tutorials for my sister while freezing in a Newfoundland outport back in 1985.

### Tools

I got up and running with dmd quite quickly. Installation on Windows 10 and FreeBSD was straightforward. A few quick questions on the D forum, and I had everything I needed to do single- or multi-file projects. A few more questions answered on the GtkD forum and I was comfortable enough to start porting my PHP-GTK code.

But I have to say, DUB eludes me. I don’t know if it’s because Electron left me feeling like JSON files are a disgruntled engineer’s revenge plot or if it’s just the way my brain works. Perhaps if I put my mind to it, I could learn it, but since I’m getting the results I want from plain ole dmd, I’ve done no more than skim DUB’s docs up until now. Just for the record, back in my C days, make files were a caution for me, too. I eventually licked them, and if it ever becomes important enough to me to figure out DUB, I guess I will. But for now, my heart’s not in it.

Finding a text editor that supports D (and especially GtkD) syntax highlighting rather than—as a few people on the forum stated—supporting C and getting ‘good enough’ support for D, led me to abandon the search and roll my own. So far, I’ve done more-or-less complete highlighters for PSPad and CodeBlocks. To be fair, they both support D out of the box, but not GtkD which is important to me, mostly as an aid to remembering module names.

I haven’t even looked at other tools. I’m dimly aware of some other sub-forums for what look like other tools, but to be honest I haven’t read them. As I said, dmd does the job, so I’m satisfied for now.

### Conclusion

And that’s a quick summary of my first two months with D. On the one hand, I don’t really understand some of D’s paradigms, but on the other, the ones I do relate to are meeting my expectations. The only two things I haven’t found yet are:

• information about how I would go about packaging a D app (with GtkD) for distribution, and
• how to build on one platform for distribution on another (if that’s even possible).

I’m willing to put in some time on this and eventually get gtkDcoding.com off the ground as my way of giving back. It’s been a long time since those frozen weeks with a Coleco Adam when I tried to explain BASIC to my thirteen-year-old sister (who is now a paramedic and doesn’t care any more). And I must say, I’m as excited now about D and GtkD as I was about BASIC and general computing way back then. Being retired, I have the time to pursue it and I’m looking forward to becoming a regular member of the D community.

And in case you don’t know what a Dutch angle is, I urge you to watch some episodes of Batman from the 1960’s. See how the camera tilts when things go wrong for our heroes? That’s a Dutch angle.

## Bio

I was inspired to learn programming while vacationing in a frozen Newfoundland outport in April of 1985. It started as a desperate attempt to keep from stripping down to my shorts and disappearing into a blizzard, but became a lifelong passion along with acting, writing, and music. In keeping with this non-typical start, it was in art college where I learned my first serious programming language (6502 assembly) and later it was on a job as a technical writer and artist that I wrote my first serious code for a client, an online mortgage calculator for a credit union in British Columbia. The culmination of my programming career was finishing the PHP-GTK app, Corkboard, a writer’s tool for story planning. (Don’t bother looking. I couldn’t come up with a distribution system, so it languishes here on a back-up drive.)

Since dropping out of high school in 1972, I’ve made a living as a taxi driver, musician, screenwriter, technical writer, artist, sound reinforcement equipment salesman, and biology lab technician among other things. I’ve also made money acting, programming, and promoting concerts. I retired from Statistics Canada in 2010 and have since divided my time between acting, writing lame novels, pursuing the elusive X-Plat beast, and keeping house for my wife of 33 years.

# How an Engineering Company Chose to Migrate to D

Bastiaan Veelo is the lead developer of a specialised program for the
computer aided geometric design of ship hulls called Fairway, for the
company SARC in the Netherlands.

Imagine there is this little-known programming language in which you enjoy programming in your free time. You know it is ready for prime time and you dream about using it at work everyday. This is the story about how I made a dream like that come true.

## My early acquaintance with D

Back when “google” was not yet a common verb, I was doing a web search for “parsing C++”. The reason was that writing a report for an assignment had derailed into writing a syntax highlighter for noweb using bison and flex, and I found out firsthand that C++ is not easy to parse. That web search brought up this page (present version) with an overview of the D Programming Language, and the following statement has had me hooked ever since:

D’s lexical analyzer and parser are totally independent of each other and of the semantic analyzer. This means it is easy to write simple tools to manipulate D source perfectly without having to build a full compiler. It also means that source code can be transmitted in tokenized form for specialized applications.

“Genius,” I thought, “here we have someone who knows what he’s doing.” This is representative of the pragmatic professionalism that still radiates from the D community, and it combines with an unpretentious flair that makes it pleasant to be around. This funny quote decorated its homepage for many years:

“Great, just what I need.. another D in programming.” – Segfault

Nevertheless, I didn’t have many opportunities to use the language and I largely remained sitting on the fence, observing its development.

## Programming professionally

With mostly academic programming experience, I started programming professionally in 2006 for SARC, a Dutch engineering company serving the maritime industry. Since the early ’80s they have been developing software for ship design and onboard loading calculations, which today amounts to roughly half a million lines of code. I think their success can partly be attributed to their choice of programming language: Extended Pascal (the ISO 10206 standard, not one of the many proprietary extensions of Pascal).

Extended Pascal was a great improvement over ISO 7185 Pascal. Its compiler, by Prospero Software from England, was fast and well documented. The language is small enough and its syntax appropriately verbose to make engineering professionals quickly productive in programming. Personally though, I spent most of my time programming in C++, modernizing their system for computer aided design of ship hulls using Qt and Coin3D.

## When your company outlives a programming language

Although selecting an ISO standard in favor of a proprietary Pascal dialect seemed wise at the time, it is apparent now that the company has outlived the language. Prospero Development Software Ltd was officially dissolved 15 years ago. Still, its former director, Tony Hetherington, continued giving support many years after, but he’d be close to 86 years old now and can no longer be reached. Its website is gone, last archived in 2013. There’s GNU Pascal, which also supports ISO 10206, but that project has stopped moving and long ago lost synchrony with gcc. Although there is no immediate crisis, it is clear that something needs to happen sometime if the company wants to continue its activities in the coming decades.

## Changing the odds

A couple of years ago, I secretly started playing with the fantasy of replacing Extended Pascal with D. Even though D’s syntax is somewhat different from Pascal, it shares at least four important similarities: support for nested functions, boundary checking, modules, and compilation speed. In addition, it has many traits that make the language attractive to engineers: good focus on performance and numerics, garbage collection, dynamic arrays, easy parallelization, understandable templates, contract programming, memory safety, unit tests, and even wysiwyg strings and formatted numerals. D’s language features encourage experimentation, which resonates well with engineers.

So I wondered what I could do to highlight D’s significance to my employer and show it’s an attractive language to switch to. I thought I could make a compelling case if I could write a parser in D that would take Extended Pascal source and transpile it to D source. At least I would have fun trying!

So I went over to code.dlang.org to see if there were any D alternatives to flex and bison. There, I found Pegged, and instantly the fun began. Pegged combines the functionality of flex and bison in one incredibly easy to use package, for which its creator Philippe Sigaud obviously enjoyed writing excellent documentation. Nowadays, Pegged is part of the D language tour and you can try it out on-line without having to install a thing. The beauty is that the grammar from the Extended Pascal language specification maps almost linearly to the PEG from which Pegged generates the parser. For this it makes heavy use of D’s generic programming capabilities and compile-time function evaluation — it can generate a parser at compile time if you want it to!

However, it wasn’t smooth sailing all along. As I was testing D, I suddenly found myself being tested as well. I learned the hard way that there is a phenomenon called left-recursion, from which a PEG parser typically cannot break out of. And the Extended Pascal grammar is left-recursive in several ways. Consequently, I spent many evenings and weekends researching parsing theory, until eventually I managed to extend Pegged with support for all kinds of left-recursion! From one thing came another, and I added longest match alternation, case insensitive literals, the toHTML() method for dynamically browsing the syntax tree, and a tracer for logging the parsing process.

Obviously, I was having fun. But more importantly, I was demonstrating that the D programming language is accessible enough that a naval architect can understand other people’s code and expand it in non-trivial ways. The icing on the cake came when I was asked to present my experiences at DConf 2017 in Berlin, which you can watch here (and here’s the extra bit I presented at lunch time for the livestream audience).

At this time, I was able to automatically translate the following trivial example:

program hello(output);

begin
writeln('Hello D''s "World"!');
end.

into D:

import std.stdio;

// Program name: hello
void main(string[] args)
{
writeln("Hello D's \"World\"!");
}

# Language competition

Having come this far, the founder of SARC agreed that it was time to investigate the merits of various alternative programming languages. We would do a thorough and objective comparison based on trial translations of a comprehensive set of language features. Due to the amount of manual labor that this requires, we had to drastically prune the space of programming languages in an initial review round. Note that what I am about to present does not declare which programming language is the best in our industry. What we are looking for is a language that allows an efficient transition from Extended Pascal without interrupting our business, and which enables us to take advantage of modern insights and tools.

In the initial review round we looked at general language characteristics. Here I’ll just highlight what fell through the sieve and why.

Performance is important to us, which is why we did not consider interpreted languages. C++ is in use for one component of our software, but that was written from the ground up. We feel that the options for translation are not favorable, that its long compile times are a serious hindrance to productivity, and that there are too many ways in which one can shoot one’s self in the foot. We cannot require our expert naval architects to also become experts in C++.

Nowadays, whenever D is publicly evaluated, the younger languages Go and Rust are often brought up as alternatives. Here, we need not go into an in-depth comparison of these languages because both Rust and Go lack one feature that we rely on heavily: nested functions with access to variables in their enclosing scope. Solutions for eliminating nested functions, like bringing them into global scope and passing extra variables, or breaking files up into smaller modules, we find unattractive because it would complicate automated translation, and we’d like to preserve the structure and style of our current code. GNU C does offer nested functions, but it is a non-standard extension and it has been predicted that many will move away from C due to its unsafe features. After this initial pruning, three languages remained on our shortlist: Free Pascal, Ada and D.

As a basis for our detailed comparison, we wrote fifteen small programs that each used a specific feature of Extended Pascal that is important in our current code base. We then translated those programs into each language on our shortlist. We kept a simple score board on how well these features were represented in each language: +1 if the feature is supported or can be implemented, 0 if the lack of the feature can be worked around, and -1 if it can’t. This is what came out of that evaluation:

Arrays beginning at arbitrary indices +1 +1 +1
Sets 0 0 +1
Schema types 0 0 +1
Types with custom initial values -1 0 +1
Classes +1 +1 +1
Casts +1 +1 +1
Protection against use of dangling pointers -1 +1 +1
Thread safe memory [de]allocation +1 +1 +1
Calling into Windows API +1 +1 +1
Forwarding Windows callbacks to nested functions +1 +1 +1
Speed of calculations +1 +1 +1
Calling procedures written in assembly +1 0 +1
Calling procedures in a DLL +1 +1 +1
Binary compatibility of strings 0 +1 +1
Binary compatible file i/o -1 0 0
Score 6 10 14

So, Free Pascal is the only candidate with negative scores, Ada positions itself in the middle, and D achieves an almost perfect score. Not effortlessly, though; we’ll talk about some of the technical challenges later. Because Free Pascal is, like D, fully Open Source and written in itself, extending the language and filling in the gaps is theoretically possible. Although some of its deficiencies could certainly be resolved that way, others would be quite complicated and/or unlikely to be accepted upstream.

We also estimated the productivity of the languages. Free Pascal scored high because it is closest to what we are used to. Despite its dissimilar syntax, D scored high because of its expressiveness and flexibility. Ada scored lowest because of its rigidity and because of the extra work the programmer has to put in (most importantly casts and conversions). Ada is more verbose than Pascal which was disliked by some of us because it can somewhat obscure the essence of what a piece of code tries to express, and frequently the code became not only verbose but cryptic, which was unanimously disliked.

Third, we estimated the future prospects and the advantages each language could bring to the table. Although Free Pascal has a more active community than we expected it to have, we do not see great potential for growth. Ada is renowned for its support for writing reliable code (although it has no monopoly in that field) but it does come at a cost and requires real effort. D has a dynamic and open community, supports both script-like productivity and high performance, includes various features for writing reliable software (approaching Ada but at a much lower cost), and offers some unique advanced features with which wonders can be accomplished.

Finally, we estimated the effort of translation. Although Free Pascal is very similar to Extended Pascal, missing features pose a real problem and would require a high degree of manual translation and rewriting. Although p2ada exists, it only works partially in our case and does not fully support Extended Pascal. Because Ada frequently requires additional code (casting to the correct type, pulling in a package, instantiating a generic type, adding a pragma, splitting up Put_Lines etc.), writing or extending a reliable transpiler into Ada would be more difficult than doing the same into D.

# Selecting a winner

I gave away the winner in the title, but we landed at that conclusion as follows. Ada was the first language to be dropped. We really felt that the extra work that the programmer has to put in is a brake on productivity and creativity. Although it barely played a role in our evaluation, illustrative is the difference between the Ada and D equivalents to the Expressive C++17 Challenge. The D solution is both concise and expressive, the Ada solution is hardly expressive and consists of more lines than I want to write or read. Also of secondary importance, but difficult to ignore, is the difference between the communities surrounding the languages, which in Ada’s case is AdaCore Support, who has no problems demanding annual five-figure subscription fees.

Although akin to our current language, Free Pascal was mainly dropped due to its porting challenges and our estimation that its potential is lower and its future outlook is less optimistic than that of D. If we were to choose Free Pascal, we would basically invest a lot of effort only to arrive at a technological solution that we felt would be of lower quality than we currently have.

And that’s were I saw a dream come true: A clap on the table by the company founder and it was decided to commit to the effort of bringing twenty-five years worth of Extended Pascal code to D!

# What makes a difference

In short, my experience is that if a feature is not present in the language, D is powerful enough that the feature can be implemented in a library. Translating each sample program by hand has really helped to focus on replicating functionality, leaving the translation process for later concern. This has led to writing a compatibility library with types and functions that are vital for the conversion. Now that equivalents are known and the parser is done, I just have to implement code generation.

Below follows another example that currently translates automatically and executes identically. It iterates over a fixed length array running from 2 to 20 inclusive, fills it with values, prints the memory footprint and writes it to binary file:

program arraybase(input,output);

type t = array[2..20] of integer;
var a : t;
n : integer;
f : bindable file of t;

begin
for n := 2 to 20 do
a[n] := n;
writeln('Size of t in bytes is ',sizeof(a):1); { 76 }
if openwrite(f,'array.dat') then
begin
write(f,a);
close(f);
end;
end.

Transpiled to D (or should I say Dascal?) and post-processed by dfmt to fix up formatting:

import epcompat;
import std.stdio;

// Program name: arraybase
alias t = StaticArray!(int, 2, 20);

t a;
int n;
Bindable!t f;

void main(string[] args)
{
for (n = 2; n <= 20; n++)
a[n] = n;
writeln("Size of t in bytes is ", a.sizeof); // 76
if (openwrite(f, "array.dat"))
{
epcompat.write(f, a);
close(f);
}
}

Of course this is by no means idiomatic D, but the fact that it is recognizable and readable is nice, especially for my colleagues who will have to go through an unusual transition. By the way, did you notice that code comments are preserved?

One very-nice-to-have feature is binary file compatibility; In fact it may have been the killer feature, without which D might not have been so victorious. The case is that whenever a persistent data structure is extended in our software, we make sure that we can still read and convert that structure from its prior format. That way, if a client pulls out an old design from its archives and runs it through our current software, it will still work without the user even being aware that conversion occurs, possibly in multiple steps. Not having to give up that ability is very attractive.

But it wasn’t easy to get there. The main difficulty is the difference in how strings are represented in D and the Prospero implementation of Extended Pascal, in memory and on file. This presented the challenge of how to preserve binary compatibility in file I/O with data structures that contain string members.

## Strings

In Prospero Extended Pascal, strings are implemented as a schema type, which is a parameterized type that can be used in the following ways:

type string80 = string(80);
var str1 : string80;
str2 : string(60);
procedure foo(s : string);

This defines string80 to be an alias for a string type discriminated to have a capacity of 80 characters. Discriminated string variables, like str1 and str2, can be passed to functions and procedures that take undiscriminated strings as arguments, like foo, which thereby work on strings of any capacity. In memory, str1 is laid out as a sequence of 80 chars, followed by a ushort that encodes the length of the string. I say encodes because a shorter string is padded with \0s up to the capacity and the ushort actually contains the length of that padding. This way, when a pointer to the string is passed to a C function and the contents of the string occupy its full capacity, the 0 in the padding length doubles as the terminating \0 of the C string.

My first thought was to mimic this data representation with a D template. But that would require procedures like foo to be turned into templates as well, which would escalate horribly into template bloat, a problem with multiple string arguments and argument ordering, and would complicate translation. Besides, schema types can also be discriminated at run time, which does not translate to a template.

Could some sort of inheritance scheme be the solution? Not really, because instances of D classes live on the heap, so a string embedded in a struct would just be a pointer instead of the char array and ushort.

But binary layout is actually only relevant in files, and in a stroke of insight I realized that this must be why user-defined attributes, or UDAs, exist. If I annotate the string with the correct capacity for file I/O, then I can just use native D strings everywhere, which genuinely must be the best possible translation and solves the function argument issue. Annotation can be done with an instance of a struct like

struct EPString
{
ushort capacity;
}

The above Pascal snippet then translates to D like so:

@EPString(80) struct string80 { string _; alias _ this; }
string80 str1;
@EPString(60) string str2;
void foo(string s);

Notice how the string80 alias is translated into the slightly convoluted struct instead of a normal D alias, which would have looked like

@EPString(80) alias string80 = string;
</code>

Although that compiles, there is no way to retrieve the UDA in that case because plain alias does not introduce a symbol. Then hasUDA!(typeof(str1), EPString) would have been equivalent to hasUDA!(string, EPString) which evaluates to false. By using the struct, string80 is a symbol so typeof(str1) gives string80, and hasUDA!(string80, EPString) evaluates to true in this example.

There is one side effect that we will have to learn to accept, and that is that taking a slice of a string does not produce the same result in D as it does in Extended Pascal. That is because string indices start at 1 in Extended Pascal and at 0 in D. My strategy is to eliminate slices from the source and replace them with a call to the standard substr function, which I can implement with index correction. Finding all string slices can be accomplished with a switch in the transpiler that makes it insert a static if to test if the slice is being taken on a string, and abort compilation if it is. (Arrays are transpiled into a custom array type that handles slices and indices compatibly with Extended Pascal.)

## Binary compatible file I/O

Now, to write structs to file and handle any embedded @EPString()-annotated strings specially, we can use compile-time introspection in an overload to toFile that acts on structs as shown below. I have left out handling of aliased strings for clarity, as well as shortstring, which is a legacy string type with yet a different binary format.

void toFile(S)(S s, File f) if (is(S == struct))
{
import std.traits;
static if (!hasIndirections!S)
f.lockingBinaryWriter.put(s);
else
// TODO unions
foreach(field; FieldNameTuple!S)
{
// If the member has itself a toFile method, call it.
static if (hasMember!(typeof(__traits(getMember, s, field)), "toFile") &&
__traits(compiles, __traits(getMember, s, field).toFile(f)))
__traits(getMember, s, field).toFile(f);
// If the member is a struct, recurse.
else static if (is(typeof(__traits(getMember, s, field)) == struct))
toFile(__traits(getMember, s, field), f);
// Treat strings specially.
else static if (is(typeof(__traits(getMember, s, field)) == string))
{
// Look for a UDA on the member string.
static if (hasUDA!(__traits(getMember, s, field), EPString))
{
enum capacity = getUDAs!(__traits(getMember, s, field), EPString)[0].capacity;
static assert(capacity > 0);
writeAsEPString(__traits(getMember, s, field), capacity, f);
}
else static assert(false, Need an @EPString(n) in front of  ~ fullyQualifiedName!S ~ . ~ field );
}
// Just write other data members.
else static if(!isFunction!(__traits(getMember, s, field)))
f.lockingBinaryWriter.put(__traits(getMember, s, field));
}
}


At the time of writing, I still have work to do for unions, which are used in the translation of variant records (including considering the use of one of the seven existing library solutions 1, 2, 3, 4, 5, 6, 7).

Currently, detecting unions is a bit involved . Also, there is a complication in the determination of the size of a union when the largest variant contains strings: the D version of that variant may not be the largest because D strings are just slices. I’ll probably work around this by adding a dummy variant that is a fixed size array of bytes to force the size of the union to be compatible with Extended Pascal. This is the reason why D scored a mere 0 in file format compatibility. It is amazing what D allows you to do though, so I may be able to do all of that automatically and award D a perfect score retroactively. On the other hand, it is probably easiest to just add the dummy variant in the Pascal source at the few places where it matters and be done with it.

# The way forward

Obviously, this is long term planning. It has taken years to grow into D; it will possibly take a year, and probably longer, to migrate to D. Unless others turn up who are in the same boat as us (please contribute!) it’ll be me who has to row this ship to D-land and I still have my regular duties to attend to. My colleagues will continue to develop in Extended Pascal as usual, and once my transpiler is able to translate all or almost all of it, we will make the switch to D overnight. From then on, we’ll be in it for the long run. We trust to be with D and D to be with us for decades to come!

# User Stories: Funkwerk

The deadline for the early-bird registration for DConf 2018 in Munich is coming up on March 17th. The price will go up from $340 to$400. If you’d like to go, hurry and sign up to save yourself \$60. And remember, the NH Munich Messe hotel, the conference venue, is offering a special deal on single rooms plus breakfast for attendees.

A few of the DConf attendees are coming from a local company called Funkwerk. They’re a D shop that we’ve highlighted here on this blog in a series of posts about their projects (you’ll see one of their products in action if you take the subway or local train service in Munich).

In this post, we cap off the Funkwerk series with the launch of a new feature we creatively call “User Stories”. Now and again, we’ll publish a post in which D users talk of their experiences with D, not about specific projects, but about the language itself. They’ll tell of things like their favorite features, why they use it, how it has changed the way they write code, or anything they’d like to say that expresses how they feel about programming in D.

For this inaugural post, we’ve got three programmers from Funkwerk. First up, Michael Schnelle talks about the power of ranges. Next, Ronny Spiegel tells why generated code is better code. Finally, Stefan Rohe enlightens us on Funkwerk’s community outreach.

## The power of ranges

Michael Schnelle has been working as a software developer for about 5 years. Before starting with D 3 years ago, he worked in (Web)Application Development, mostly with Java, Ruby on Rails, and C++, and did Thread Modeling for Applications. He enjoys coding in D and likes how it helps programmers write clean code.

In my experience, no matter what I am programming, I always end up applying functions to a set of data and filter this set of data. Occasionally I also execute something with side effects in between. Let’s look at a simplified use case: the transformation of a given set of data and filtering for a condition afterwards. I could simply write:

foreach(element; elements) {
auto transformed = transform(element);
if (metCondition(transformed) {
results ~= transformed
}
}

Using the power from std.algorithm, I can instead write:

filter!(element => metCondition(element))
(map!(element => transform(element))(elements));

At this point, we have a mixture of functional and object-oriented code, which is quite nice, but still not quite as readable or easy to understand as it could be. Let’s combine it with UFCS (Uniform Function Call Syntax):

elements.map!(element => element.transform)
.filter!(element => element.metCondition);

I really like this kind of code, because it is clearly self-explanatory. The foreach loop, on the other hand, only tells me how it is being done. If I look through our code at Funkwerk, it is almost impossible to find traditional loops.

But this only takes you one step further. In many cases, there happen to be side effects which need to be executed during the workflow of the program. For this kind of thing, the library provides functions like std.range.tee. Let’s say I want to execute something external with the transformed value before filtering:

elements
.map!(element => element.transform)
.tee!(element => operation(element))
.filter!(element => element.metCondition)
.array;

It is crucial that operations with side effects are only executed with higher-order functions that are built for that purpose.

int square(int a) { writefln("square value"); return a*a; }

[4, 5, 8]
.map!(a => square(a))
.tee!(a => writeln(a))
.array;

The code above would print out the square value six times, because tee calls range.front twice. It is possible to avoid this by using functions like std.algorithm.iteration.cache, but in my opinion, the nice way would be to avoid side effects in functions that are not meant for that.

In the end, D gives you the possibility to combine the advantages of object-oriented and functional programming, resulting in more readable and maintainable code.

## Generated code is better code

Ronny Spiegel has worked as a professional software developer for almost 20 years. He started out using C and C++, but when he joined Funkwerk he really started to love the D language and the tools it provides to introspect code and to automate things at compile time.

In a previous blog post, I gave a short overview of the evolution of the accessors library. As you might imagine, I really like the idea of using the compiler to generate code; in the end this usually results in less work for me and, as a direct result, causes fewer errors.

The establishment of coding guidelines is crucial for a team in order to create maintainable software, and so we have them here at Funkwerk. There is a rule that every value object (or entity) has to implement the toString method in order to provide diagnostic output. The provided string shall be unambiguous so that it’s more like Python’s __repr__ than __str__.

Example:

StationMessage(GeneralMessage(4711, 2017-12-12T10:00:00Z), station="BAR", …)

The generated string should follow some conventions:

• provide a way to uniquely reconstruct data from a string
• continue with any potential superclasses
• list all fields providing their name and value separated by a comma
• be compact but still human readable (for developers)
• skip the name where it matches the type (e.g. a field of type SysTime is called time)
• skip the name if the field is called id (usually there’s an IdType used for type safety)
• there’s some special output format defined for types like Date and SysTime
• Nullable!T’s will be skipped if null etc.

To format output in a consistent manner, we implemented a SinkWriter wrapping formattedWrite in a way that follows the listed conventions. If this SinkWriter is used everywhere, this is the first step to fully generate the toString method.

Unfortunately that’s not enough; it’s very common to forget something when adding a new field to a class. Today I stumbled across some code where a field was missing in the diagnostics output and that led to some confusion.

Using (template) mixins together with CTFE (Compile Time Function Execution) and the provided type traits, D provides a powerful toolset which enables us to generate such functions automatically.

We usually implement an alternative toString method which uses a sink delegate as described in https://wiki.dlang.org/Defining_custom_print_format_specifiers. The implementation is a no-brainer and looks like this:

public void toString(scope void delegate(const(char)[]) sink) const
{
alias MySelf = Unqual!(typeof(this));

sink(MySelf.stringof);
sink("(");

with (SinkWriter(sink))
{
write("%s", this.id_);
write("station=%s", this.station_);
// ...
}

sink(")");
}

This code seems to be so easy that it might be generalized like this:

public void toString(scope void delegate(const(char)[]) sink) const
{
import std.traits : FieldNameTuple, Unqual;

alias MySelf = Unqual!(typeof(this));

sink(MySelf.stringof);
sink("(");

with (SinkWriter(sink))
{
static foreach (fieldName; FieldNameTuple!MySelf)
{{
mixin("const value = this." ~ fieldName ~ ";");
write!"%s=%s"(fieldName, value);
}}
}

sink(")");
}

The above is just a rough sketch of how such a generic function might look. For a class to use this generation approach, simply call something like

mixin(GenerateToString);

inside the class declaration, and that’s it. Never again will a field be missing in the class’s toString output.

Generating the toString method automatically might also help us to switch from the common toString method to an alternative implementation. If there will be more conventions over time, we will only have to extend the SinkWriter and/or the toString-template, and that’s it.

As a summary: Try to generate code if possible – it is less error prone and D supports you with a great set of tools!

## Funkwerk and the D-Community

Stefan Rohe started the D-train at Funkwerk back in 2008. They have loved DLang since then and replaced D1-Tango with D2-Phobos in 2013. They are strong believers in open source and local communities, and are thrilled to see you all in Munich at DConf 2018.

Funkwerk is the largest D shop in south Germany, so we hire D-velopers, mainly just through being known for programming in D. In order to give a little bit back to the D community at large and help the local community grow, Funkwerk hosted the foundational edition of the Munich D Meetup.

### The local community is important …

The meetup was founded in August 2016, 8 years after the first line of D code at Funkwerk was written. Since then, the Meetup has grown steadily to ~350 members. At that number, it is still not the biggest D Meetup, but it is the most visited and the most active. It provides a chance for locals in Munich to interact with like-minded D-interested people each month. And with an alternating level of detail and a different location each month, it stays interesting and attracts different participants.

### … and so is the global community

To engage with the global community, Funkwerk is willing to open source some of its general-purpose D libraries. They can all be found under github.com/funkwerk, and some are registered in the DUB registry.

To mention are:

• accessors – a library to auto generate getters and setters with UDAs
• depend – a tool that checks actual import dependencies against a UML model of target dependencies
• d2uml – reverse engineering of D source code into PlantUML class outlines

Feel free to use these and let us know how you like them.