Ruminations on D: An Interview with Walter Bright

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Joakim is the resident interviewer for the D Blog. He has also interviewed members of the D community for This Week in D and is responsible for the Android port of LDC.

d6Walter Bright is the creator and first implementer of the D programming language. He was an early developer of C++ compilers starting from the mid-’80s, including the first C++ compiler to translate source code directly to object code without using C as an intermediate, and has written compilers for ABEL, C, Java, and Javascript. He believes he is the only person to have written a full C++98 compiler by himself. Empire, one of the first computer strategy games, was written by Walter at Caltech. Before getting into computer programming full-time, Walter worked for Boeing from 1979-1982 on the 757’s flight controls, particularly the stabilizer trim gearbox and system and the elevators.

Joakim: D appears to be picking up speed. The fourth straight DConf took place earlier this year, Wired wrote a nice article a couple years ago, Gartner ranked it in the top 20 languages soon afterwards, and downloads of the reference compiler, DMD, average 1200 per day in recent years. Talk about the current popularity and status of the language and what you’re doing to take it to the next level.

Walter: I don’t worry too much about that. I spend my efforts making D the best language possible, and let the metrics take care of themselves. It’s like being a CEO; he shouldn’t be sweating the stock price, he should be working on making money for the company, then the stock price will take care of itself.

There’s always a stack of things to do, more or less with the most important one on the top, and I pop the top off and work on it, mostly the one with the maximum benefit/cost ratio. The ordering in the stack changes all the time. If an item has others strongly interested in working on it, I defer to them.

I get the jobs that nobody else wants to do. ūüôā Regression fixes for complex problems is a big one. It’s much more fun designing new stuff than maintenance on the existing stuff, dealing with technical debt, etc.

Joakim: In the last year, @nogc and interfacing with C++ have been at the top of your stack. Why? You always used to say that interfacing more with C++ would be too much work.

Walter: It became clear that the garbage collector wasn’t needed to be embedded in most things, that memory allocation could be decided separately from the algorithm. Letting the user decide seemed like a great way forward.

As for C++, I figured out a way to support it that avoided the problems I thought were impractical to deal with. I did a talk about it PDF slides here– and a Rust user wrote up a partial transcript.

Since that talk, I’ve come up with a solution for exceptions. C++ can throw/catch exceptions of any type. But few people do anything other than throw/catch a reference to a class, like std::exception. All D needed to do was allow throw/catch of a class marked with the extern (C++) attribute. Throw/catch of other types remains unsupported, and most likely will remain unsupported.

In order to make that work, the custom exception handling code in the code generation and the library had to change to work with the DWARF exception handling mechanism. That turned out to be a fair amount of work, as it is rather under-documented. But it’s pretty simple for the user.

Joakim: What do you plan on working on in D for the rest of the year? Work on @nogc and C++ support seems to be ongoing and you’ve tried to increase the use of ranges in the standard library for some time now. Anything else? What is the status of those efforts?

Walter: Those are still high-priority and ongoing efforts. But also we’re flexible; if a major opportunity comes up that needs us to push in a different direction, we’ll adapt. The pervasive use of ranges is advancing rapidly, I’ve been very pleased with the results.

The current focus is on improving memory safety. It’s become more and more important to have verifiable memory safety in a programming language, as the expenses involved when unsafe code is exploited in order to install malware become greater and greater. D has always supported memory safety, but recently we’ve embarked on a much more comprehensive review of memory safety in the core language and are making changes to close the gaps.

Joakim: How much time do you spend on D and what is your daily routine?

Walter: I work full time on D. I probably spend half my time working on the language, and the other half helping others with it, discussing things, doing interviews (!), writing articles, doing conferences, etc.

Joakim: You were 42 when you started working on D and I guess it is the first language you designed? Talk about why you started working on it so late–you were probably older then than most of the D contributors now–and what insights your experience gave you that your younger self or other young contributors may not have.

Walter: ABEL is the first language I designed, and was a solid success for Data I/O. It’s obsolete now, because the electronic devices it was aimed at are now obsolete, but it had a great run for 10 years or so.

Having been writing compilers my whole career, doing tech support for them, and following the various changes in the languages inevitably gave me much insight on what worked and what didn’t. Probably the biggest thing is that simpler is better. But making something simple is actually quite difficult. Anybody can (and does) design a complex solution, but few can see through the complexity to find the underlying simplicity.

Many successful languages were designed by older engineers.

Joakim: Please give some examples of such simplicity and how you were able to find it.

Walter: We nailed it with arrays (Jan Knepper’s idea), the basic template design, compile-time function execution (CTFE), and static if. I have no idea what the thought process is in any repeatable manner. If anything, it’s simply a dogged sense that there’s got to be a better way. It took me years to suddenly realize that a template function is nothing more than a function with two sets of parameters –compile time and run time–and then everything just fell into place.

I was more or less blinded by the complexity of templates such that I had simply missed what they fundamentally were. There was also a bit of the “gee, templates are hard” that predisposes one to believe they actually are hard, and then confirmation bias sets in.

I once attended a Scott Meyers presentation on type lists. He took an hour to explain it, with slide after slide of complexity. I had the thought that if it was an array of ints, his presentation would be 2 minutes long. I realized that an array of types should be equally straightforward.

With CTFE, we just went straight in the front door from asking the question “why can’t we execute this function at compile time, just like constant folding?” I did the initial one just by extending the constant folding logic. Don Clugston took it much further, but at its heart it’s still a modified dwarf. Stefan Koch is currently working on making a real interpreter out of it.

Joakim: Why do you think there hasn’t been a killer app for D yet? For example, Ruby was kind of an obscure language for a dozen years till Rails propelled it into the spotlight.

Walter: I suspect the age of the killer app is behind us. There is so much software existing and being written, for every imaginable purpose, that it’s hard to believe there will even be another killer app of any sort. Of course, predictions are notoriously unreliable.

Joakim: D has a unique approach in the compiled languages market, being mostly community-developed without a major corporate sponsor. C++ had Bell Labs, Rust has Mozilla, Go has Google; all have full-time paid devs on the language, even if they also take open-source contributions. Do you think this is a problem and is D being left behind?

Walter: We recently started a D foundation which will make it a lot easier for corporations to sponsor D.

Joakim: Do you make money off D? I know you’ve contracted with Facebook to write a C++ linter, Flint, and preprocessor, Warp, and that you work with Sociomantic and other companies using D.

Walter: I do some paid consulting work for D, but am careful not to take on so much that it interferes with working on D itself.

Joakim: Do you write much code in D outside of the standard library? If so, talk about recent stuff you’ve written and how the experience has been, plus a bit about using some of the new features.

Walter: D consumes so much of my efforts, there’s not much time to write other D apps other than smallish utilities. Currently, of course, the D compiler front end itself is now in D. But it’s been translated from C++, so isn’t idiomatic D.

Joakim: OK, so I guess Warp is the largest program you’ve written in D lately, about 10 klocs. Can you talk about the experience of actually coding that in D, as opposed to C/C++? What stood out for you?

Walter: What stood out is the speed with which it went together, and the remarkably small number of bugs that surfaced in it after it was released. I credit the extensive use of unit tests for the latter.

Having written another preprocessor before certainly helped, and it would be hard to tease that out as a separate effect. But I still believe that unit testing made the difference, because the way the preprocessor worked was very different from the one I’d done before.

What stood out with D was the ease of changing the data structures to try different ways, compared with doing this in other languages.

Joakim: When you think back to your vision for D as you were starting out in 1999, does it at all resemble that today? Anything big missing that you wanted back then?

Walter: D is far more advanced than what I thought of 15 years ago. Programming language ideas have certainly advanced since then, and D along with it.

D originally wasn’t going to have templates at all, based on my earlier experience with them. But finding a simple way to do them changed everything – even to the point where well over half of a modern D program is templates!

The idea of ranges slowly evolved over time, we’re still learning how to do it right.

bit as a basic type was unworkable. complex as a basic type turned out to be pointless (it works better as a library type). Auto-decoding of UTF-8 to code points turned out not nearly as useful as expected.

Transitive const was a leap of faith, and is consistently overlooked by other languages looking to adopt D features. I have a lot of faith in transitive const, and it is already paying off in making it possible to have pure functions, a key feature for modern programming.

Inside D Version Manager

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In his day job, Jacob Carlborg¬†is a Ruby backend developer for¬†Derivco¬†Sweden, but he’s been using D on his own time since 2006. He is the maintainer of numerous open source projects, including DStep, a utility that generates D bindings from¬†C and Objective-C headers, DWT, a port of the Java GUI library SWT, and the topic of this post, DVM. He implemented native Thread Local Storage support for DMD on OS X and contributed, along with Michel Fortin, to the integration of Objective-C¬†in D.

D Version Manager (DVM), is a cross-platform tool that allows you to easily download, install and manage multiple D compiler versions. With DVM, you can select a specific version of the compiler to use without having to manually modify the PATH environment variable. A selected compiler is unique in each shell session, and it’s possible to configure a default compiler.

The main advantage of DVM is the easy downloading and installation of different compiler versions. Specify the version of the compiler you would like to install, e.g. dvm install 2.071.1, and it will automatically download and install that version. Then you can tell DVM to use that version by executing dvm use 2.071.1. After that, you can invoke the compiler as usual with dmd. The selected compiler version will persist until the end of the shell session.

DVM makes it possible for the user to select a specific compiler version without having to modify any makefiles or build scripts. It’s enough for any build script to refer to the compiler by name, i.e. dmd, as long as the user selects the compiler version with DVM before invoking the script.


DVM was created in the beginning of 2011. That was a different time for D. No proper installers existed, D1 was still a viable option, and each new release of DMD brought with it a number of regressions. Because of all the regressions, it was basically impossible to always use the latest compiler, and often even older compilers, for all of your projects. Taking into consideration projects from other developers, some were written in D1 and some in D2, making it inconvenient to have only one compiler version installed.

It was for these reasons I created DVM. Being able to have different versions of the compiler active in different shell sessions makes it easy to work on different projects requiring different versions of the compiler. For example, it was possible to open one tab for a D1 compiler and another for a D2 compiler.

The concept of DVM comes directly from the Ruby tool RVM. Where DVM installs D compilers, RVM installs Ruby interpreters. RVM can do everything DVM can do and a lot more. One of the major things I did not want to copy from RVM is that it’s completely written in shell script (bash). I wanted DVM to be written in D. Because it’s written in shell script, RVM enables some really useful features that DVM does not support, but some of them are questionable (some might call them hacks). For example, when navigating to an RVM-enabled project, RVM will automatically select the correct Ruby interpreter. However, it accomplishes this by overriding the built-in cd¬†command. When the command is invoked, RVM will look in the target directory for one of the files .rvmrc or .ruby-version. If either is present, it will read that file to determine¬†which Ruby interpreter to select.

Implementation and Usage

One of the goals of DVM was that it should be implemented in D. In the end, it was mostly written in D with a few bits of shell script. Note that the following implementation details are specific to the platforms that fall under D’s Posix umbrella, i.e. version(Posix), but DVM is certainly available for Windows with the same functionality.

Structure of the DVM Installation

Before DVM can be used, it needs to install itself. This is accomplished with the command, dvm install dvm. This will create the ~/.dvm directory. It contains the following subdirectories: archives, bin, compilers, env and scripts.

archives contains a cache of downloaded zip archives of D compilers.

bin contains shell scripts, acting as symbolic links, to all installed D compilers. The name of each contains the version of the compiler, e.g. dmd-2.071.1, making it possible to invoke a specific compiler without first having to invoke the use command. This directory also contains one shell script, dvm-current-dc, pointing to the currently active D compiler. This allows the currently active D compiler to be invoked without knowing which version has been set. This can be useful for executing the compiler from within an editor or IDE, for example. A shell script for the default compiler exists as well. Finally, this directory also contains the binary dvm itself.

The compilers directory contains all installed compilers. All of the downloaded compilers are unpacked here. Due to the varying quality of the D compiler archives throughout the years, the install command will also make a few adjustments if necessary. In the old days, there was only one archive for all platforms. This command will only include binaries and libraries for the current platform. Another adjustment is to make sure all executables have the executable permission set.

The env directory contains helper shell scripts for the use command. There’s one script for each installed compiler and one for the default selected compiler.

The scripts directory currently only contains one file, dvm. It’s a shell script which wraps the dvm binary in the bin directory. The purpose of this wrapper is to aid the use¬†command.

The use Command

The most interesting part of the implementation is the use command, which selects a specific compiler, e.g. dvm use 2.071.1. The selection of a compiler will persist for the duration of the shell session (window, tab, script file).

The command works by prepending the path of the specified compiler to the PATH environment variable. This can be ~/.dvm/compilers/dmd-2.071.1/{platform}/bin for example, where {platform} is the currently running platform. By¬†prepending the path to the environment variable, it guarantees¬†the selected compiler takes precedence over any other possible compilers in the PATH. The reason the {platform} section of the path exists is related to the structure of the downloaded archive. Keeping this structure avoids having to modify the compiler’s configuration file, dmd.conf.

The interesting part here is that it’s not possible to modify the environment variables of the parent process, which in this case is the shell. The magic behind the use command is that the dvm command that you’re actually invoking is not the D binary; it’s the shell script in the ~/.dvm/scripts path. This shell script contains a function called dvm. This can be verified by invoking type dvm | head -n 1, which should print dvm is a function if everything is installed correctly.

The installation of DVM adds a line to the shell initialization file, .bashrc, .bash_profile or similar. This line will load/source the DVM shell script in the ~/.dvm/scripts path which will make the dvm command available. When the dvm function is invoked, it will forward the call to the dvm binary located in ~/.dvm/bin/dvm. The dvm binary contains all of the command logic. When the use command is invoked, the dvm binary will write a new file to ~/.dvm/tmp/result and exit. This file contains a command for loading/sourcing the environment file available in ~/.dvm/env that corresponds to the version that was specified when the use command was invoked. After the dvm binary has exited, the shell script function takes over again and loads/sources the result file if it exists. Since the shell script is loaded/sourced instead of executed, the code will be evaluated in the current shell instead of a sub-shell. This is what makes it possible to modify the PATH environment variable. After the result file is loaded/sourced, it’s removed.

If you find yourself with the need to build your D project(s) with multiple compiler versions, such as the current release of DMD, one or more previous releases, and/or the latest beta, then DVM will allow you to do so in a hassle-free manner. Pull up a shell, execute use on the version you want, and away you go.

Project Highlight: Visual D

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In the world of modern software development, a language that is not supported in any of the major Integrated Development Environments is not going to gain very much traction. For better or worse, the IDE has become a widespread and permanent fixture. Programmers are free to use any editor or environment they want in their own time, but those who write software for a living and are not self-employed are going to have to work in the environment(s) allowed by their employer. In Windows shops, that often means Visual Studio.

For the first several years¬†of D’s existence,¬†not only was there no support for it in Visual Studio, the toolchain on Windows produced binaries only in the OMF format, making them incompatible with the Microsoft tools out of the box. Today, that has changed, and both DMD and LDC can output COFF files that are compatible with the MS linker. But even before DMD got support for COFF, one community member initiated Visual D, a project to bring D to the Visual Studio environment.

Rainer Schuetze first discovered D shortly after work on D2 began.

It must have been an article in the German c’t magazine (maybe this one) that brought my attention to the D language. The meta programming features of D2 caught my eye, namely templates and compile-time function evaluation. It was refreshing to see these elements being neatly integrated into the language, not bolted on top of it with a different language as in C++.

He decided to give D a try. The road that led him to Visual D originated¬†not from¬†a longing for an IDE, but from¬†his attempts at debugging DMD’s output.

The options D had to offer on Windows were rather disappointing: the version of Microsoft’s Windbg distributed with DMD was barely usable as it was already 12 years old back then in 2008. Newer versions didn’t work at all with the executables generated by DMD. The debugger being one of the highlights of Visual Studio, I tried it, too, but anything newer than VS.NET 2003 failed miserably, with the latter being almost acceptable. It¬†didn’t stop execution on breakpoints. So it seemed that there might be just a small tweak to make debugging work considerably better with Visual Studio.

It was this desire to debug D programs that led Rainer to start his first open source project, cv2pdb. Rainer’s tool can convert CodeView format output by DMD¬†to the PDB format used by the Visual Studio debugger. Once that was working, it¬†was no great leap to want better integration with the Visual Studio environment, such as build support and code completion.

Rainer found one¬†abandoned project that had been started toward that end already, but he¬†couln’t get it functional. He also experimented with modifying an existing language service written in C#, but had trouble getting it to work with a parser written in D. In the end, he decided that spending his time with D “should not mean writing code in other languages.” So, in 2009, Visual D was born. He ran into some difficulties almost immediately, as anyone using D in those days was bound to.

At the time, you would hit a compiler bug every few hundred lines of code, but these were not the biggest obstacles on the way to getting this extension to work. Visual Studio is a Win32 application and loads its extensions dynamically as DLLs. D very much relies on Thread Local Storage (TLS) as it is the default for global variables. Under Windows this uses “implicit¬†TLS” built into the binary. Unfortunately, Windows Vista was the first version to support this for dynamically loaded DLLs, but Windows XP was still the most widely used version. It only supports TLS for the DLLs that are loaded implicitly with the application.

Eventually, after a lot of debugging, he managed to work around his Windows XP problems by tricking the system into believing a manually loaded DLL had been implicitly loaded with the application. The result of these efforts can be seen in the DRuntime modules and This implementation comes with the drawback that DLLs using it cannot be unloaded. An improved version by Denis Shelomovskii works around this. Given the decline of XP usage, the need for this will eventually fade away.

TLS wasn’t his only problem.

Another issue turned up with the interfaces that create the API between Visual Studio and its extensions: these are all COM interfaces. Fortunately, D supports COM out of the box, virtual function calls work nicely. Unfortunately, the implementation in the library is currently unusable. The allocation of a COM object (every class derived from IUnknown) is done using C’s malloc, assuming the usual approach to free this memory in the Release() function when the reference count goes down to zero. Unfortunately, this is not possible from within a member function of a D class as the invariant is still called before the function returns. That’s why the runtime implementation just leaks the memory.

As a solution, he initially implemented his own ComObject class and overloaded new. Later, when the overloading of new¬†was deprecated in the language, he switched to using a factory function for COM objects. He wasn’t finished yet.

The Visual Studio SDK provides COM interface definitions for C++ that I started to convert manually on a per case basis whenever I needed a declaration in D. This proved rather tedious as dependencies grew, so I considered converting the interface definitions automatically. These are given by Interface Definition Language (IDL) files or C++ header files.

So he created a conversion tool.

It uses a tokenizer followed by some custom conversion functions (mostly dealing with C pre-processor code) and a long list of replacement rules using the function now accessible within Visual D as Search/Replace Tokens. Each token comes with the comment preceding it, so the D files very much resemble the original files and still contain documentation, which is very helpful for the VS SDK. The conversion now works for all files of the VS SDK versions 2008 to 2015 and a selection of required files from the Windows SDKs 6.0 to 10 (about 90 header files converted, some more stubbed to be mostly empty).

Visual D has come a long way since Rainer first started working on it, but the journey is by no means complete.¬†The next release will integrate the expression evaluator of the Mago debugger with the Visual Studio debug engine, thanks to Microsoft’s publication of information about the debug engine’s extensibility. Additionally, Visual D users will gain the ability to integrate with C and C++ projects.

Just drop your D files into a C/C++ project and they compile with the rest of your application without further ado. Enjoy settings integration instead of the ancient looking project dialogs of visualdproj files. More importantly, all the additional tools like the Manifest Tool, MIDL Compiler and even Build Extensions (e.g. Assembler) are easily available to D projects, too.

These new features are available in a preview version right now. Rainer has been short on time, so the features haven’t improved much since the preview release, but once he finds the time you can be sure to see these new features in the next release.

The Origins of the D Cookbook

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Adam Ruppe is the author of D Cookbook and maintainer of This Week in D. Modules from his freely available arsd package are used throughout the D community. He is also known for his legendary DConf 2014 presentation.

dcookbookIn 2013, Packt Publishing approached me to write D Cookbook. Over the next several months, I was tasked with collecting over 100 D programming language “recipes” and writing a couple of pages about each one.

I wanted it to be a mix of practical and fun examples of what D can do over the wide variety of fields in which I have used it, each one illustrative of some language concept that the reader could use in different contexts. As such, for the majority of the book, I avoided the use of libraries, even keeping the presence of Phobos or my own modules to a minimum, allowing me to focus on the implementations of the ideas.

That said, I didn’t come up with a hundred recipes out of thin air! They came from three main sources: my arsd libraries, my support efforts on the D forums, the #D IRC channel, and Stack Overflow (it was Stack Overflow that got Packt’s attention in the first place), and a few other side projects, like my minimal D runtime (zip file).

I first saw D way back in 2002. At the time, I was still fairly new to programming and was going to download another copy of Digital Mars C++ (which I used to compile 16-bit DOS programs!) when I saw a link on the Digital Mars website about this D programming language. My first impression – upon seeing the import keyword – was that it was too Java-like and I wasn’t interested. Oh, how I regret that naive snap decision! I didn’t take my next look at D until 2006, and that time, with far more real world experience and theoretical knowledge under my belt, I quickly fell in love.

Over the next year, I wrote a game library based on SDL and OpenGL and made a few little games for myself. My D code at this point wasn’t terribly unique. Much of it was based on my existing C++ codebases. That fairly uninteresting fact would later become one of my most recurring D tips: a great deal of your C, C++, and Java knowledge can carry over directly to D! Thanks to the similarities between the languages, it’s easy to learn. While your code is unlikely to work perfectly¬†if simply copy/pasted into a .d file, porting it probably isn’t hard. I found the D language very comfortable right off the bat. Even today, I tell people with D questions to simply consider how they would do it in C++¬†and apply that existing knowledge to D. This can also work with C and C++ solutions gleaned from Internet resources like Stack Overflow.

I started working as a full-time web developer in 2008, which put a time constraint on my hobby game development efforts, but didn’t ice my love of D. Indeed, it wasn’t long at all before I worked D into my professional life by writing web libraries and eventually transitioning my¬†work projects away from PHP and onto D!

At the same time, the D language was going through a series of rapid changes. Templates and compile time function evaluation got overhauled, immutable was introduced… Most interestingly to me, compile time reflection got massively expanded and a few features like opDispatch got added. Compile time reflection in older¬†D was limited to the is expression and template tricks, but new D had __traits, making it competitive even with dynamic languages, without sacrificing D’s other strengths.

I was a very early adopter of these features and set out to discover how¬†to combine them in ways to make my work easier, to compete with the other web languages, and to just show off a little ūüôā If someone came on the forum and said D can’t do this, then I’d reply Challenge accepted.

In the following¬†years, I wrote: a DOM and JSON library, taking the most interesting ideas from Javascript into D; a web framework, realizing some dreams I had in rapidly churning out prototypes that could also survive the change process of real world development; OAuth and database libraries to support the needs of the projects; and more. One of the most interesting modules was my web.d, which takes a D class definition and builds web infrastructure around it: an automatically generated web site with CRUD forms from the static type information as well as Javascript and PHP code for API access to that functionality. This stretched D’s reflection capabilities and hit quite a few bugs on the bleeding edge, necessitating creative workarounds or alternative approaches. If I was really desperate, I’d fix the bug in DMD¬†myself!

At the same time, I was regularly seen in the D community, helping other people with their problems, and, of course, reading insights from other members of the community. Every few days, I had another tip, and was also building up a mental picture of people’s common difficulties with D.

By 2013, I had years of experience with almost every corner and combination of D. Now, you can get a good slice of that in just a few days by reading the book!