Type-safe tagged unions in the D programming language

A tagged union

Sometime you have some data that can be of type A or B. Very common use cases include decoding various file format or network protocols, communicate with memory mapped devices, some processing that can return an A or a B depending on its result, and many more.

To resolve that issue, it is common to use a union. A union is a aggregate where all members share the same memory. It is very handy, but use it wrongly and you end up badly messing up your memory. I’ll demonstrate in this article how to make this safe in D.

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A story about optimization, LLVM, and the SentinelInputRange

Developers want code that runs fast and users do too. However, it is easy to fall into the trap of early optimization; this story shows that even in non-trivial cases, modern compilers can do wonders. On the other hand, the myth of the “sufficiently smart compiler” is still very relevant. The best way is to look at what is actually generated for an example and that is what I’ll do in this article. Enter the SentinelInputRange.

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Visitor pattern revisited in D

OOP is known (or should be known) as a behavioral abstraction. It nicely solves a vast variety of problems, but enforces on the programmer an abstraction that doesn’t fit every problem. The fact is, you sometime need objects as data abstractions and not behavioral abstractions.

This is commonly the case for tools that manipulate code. A parser typically returns an AST that will be processed. The processing can be compilation, code formating, code analysis, and basically any other use related to source code manipulation.

Following regular OOP principles, manipulations have to be implemented by the AST node classes. This is a problem, because it prevents any 3rd party developer tool to reuse an existing parser and AST. The known solution is to use the Visitor pattern, which allows to dispatch in our code according to the AST node’s type.

The Visitor pattern has many known problems, to the point that many consider it an antipattern. A better solution using D has to be proposed.

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Get an exception from a segfault on linux (x86 and x86_64), using some black magic !

Linux (as other UNIXes) allow you to register handlers for signals. Here we are interested in SIGSEGV. This signal is sent to your program when you try to use a memory location you shouldn’t. Typically, when deferencing null.

Language like Java send a NullPointerException, that can be caught and you can recover from it. However, in a system language, you usually get a cryptic « segmentation fault », you cannot recover from it and cannot have any information about it outside a debugger. Let’s see how we can fix this.

As C++ and D are system languages that support exceptions, we will use this mechanism to handle SIGSEGV. I’ll do it in D in this post, but the same is doable in C++. If you understand why it work, it shouldn’t be a problem.

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Impact of 64bits vs 32bits when using non precise GC.

As ARM release its intention to go 64bits, we can be pretty much sure that almost every device will be 64bits soon, except the one on which it is unrealistic to run a garbage collector.

A garbage collector can be precise like in Java, or not, like D’s GC or Boehm’s GC for C++. It means that theses GC don’t know what it a pointer and what isn’t. Non precise GC cannot move data to compact the heap, and are also prone to false positive. On a theoretical perspective, switching from 32 bits to 64 bits should improve things, but what it is in practice ?

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A review of D2 – constness and immutablility

The D programming language is a very promising language. I propose here to do a quick review of some problems it is facing (or at least problems I’m facing when using it) and propose some solutions.

Immutability and constness in D2

I wanted to to a single, but writing it, it kept getting longer and longer. The point wasn’t to rant randomly, but also to make some proposal to improve things. So I did split it into smaller articles, and here the first one. We will discuss constness and immutability in D2.

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The hashtable closure pattern

I noticed that I was using a pattern of my own more and more. It is actually very usefull, for several reasons. It may have been used by other before me, but never saw a name for it.

This pattern is useful when you have to apply an operation dependent on parameters several times. for example when you want to apply a calculation on all elements of an array, the calculation depending on some parameters. This article will explain you that pattern and demonstrate how it can be used in Javascript, PHP and D.

This pattern can be used in any language that support first class function and hashtable, that is becoming more and more common (closure support was added in PHP as of version 5.3).

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Interfacing D with C++ : start new thread

Interfacing D with C++ is sometime tricky. In this scenario, I will show how to start a thread from C++ that execute C++ code, but in a valid D environment, so that C++ code can interact with D code safely.

This work as been done to port SFML2 to D2 language. But the technique is useful for anybody who want to start safely thread in a environment with code in both C++ and D.

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