# Djinn: A Code Generator and Templating Language Inspired by Jinja2

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Code generators can be useful tools. I sometimes use the command line version of Jinja2 to generate highly redundant config files and other text files, but it’s feature-limited for transforming data. Obviously the author of Jinja2 thinks differently, but I wanted something like list comprehensions or D’s composable range algorithms.

I decided to make a tool that’s like Jinja2, but lets me generate complex files by transforming data with range algorithms. The idea was dead simple: a templating language that gets rewritten directly to D code. That way it supports everything D does, simply because it is D. I wanted a standalone code generator, but thanks to D’s mixin feature, the same templating language works as an embedded templating language (for HTML in a web app, for example). (For more on that trick, see this post about translating Brainfuck to D to machine code all at compile time using mixins.)

# Woothee (HTTP User Agent Parser)

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I’ve written a D implementation of the Project Woothee multi-language HTTP user agent parser. Here are some notes about what it’s useful for, and a few things special about the D implementation.

# D Declarations for C and C++ Programmers

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Because D was originally created by a C++ compiler writer, Walter Bright, it’s an easy language for C and C++ programmers to learn, but there are little differences in the way declarations work. I learned them piecemeal in different places, but I’m going to dump a bunch in this one post.

# urllibparse

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TL;DR: I’ve translated Python’s urllib.parse to D for parsing, building and transforming URLs. You can get it from Gitlab.

URL handling is one of those things that most of the time can be done with a regex that mostly works. But sometimes I want a just-works tool when writing D, so I translated Python’s URL handling library. The API isn’t perfect (e.g., the url_split and url_parse distinction is a bit confusing), but it’s been tested against multiple RFCs and had plenty of real-world battle hardening.

My translation is meant to give the same output as Python does, so I’ve translated the Python test suite as well. I don’t plan to add any new features that aren’t in Python.

I hope someone else finds it useful.

# Analysing D Code with KLEE

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KLEE is a symbolic execution engine that can rigorously verify or find bugs in software. It’s designed for C and C++, but it’s just an interpreter for LLVM bitcode combined with theorem prover backends, so it can work with bitcode generated by ldc2. One catch is that it needs a compatible bitcode port of the D runtime to run normal D code. I’m still interested in getting KLEE to work with normal D code, but for now I’ve done some experiments with -betterC D.

# Profiling D's Garbage Collection with Bpftrace

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Recently I’ve been playing around with using bpftrace to trace and profile D’s garbage collector. Here are some examples of the cool stuff that’s possible.

# D as a C Replacement

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Sircmpwn (the main developer behind the Sway Wayland compositor) recently wrote a blog post about how he thinks Rust is not a good C replacement. I don’t know if he’d like the D programming language either, but it’s become a C replacement for me.

# D in the Browser with Emscripten, LDC and bindbc-sdl (translation)

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Here’s a tutorial about using Emscripten to run D code in a normal web browser. It’s uses a different approach from the Dscripten game demo and the dscripten-tools toolchain that’s based on it.

• Instead of porting the D runtime, it uses a lightweight, runtimeless -betterC build.
• It uses Docker to manage the Emscripten installation.

LDC has recently gained support for compiling directly to WebAssembly, but (unlike the Emscripten approach) that doesn’t automatically get you libraries.

You can find the complete working code on Github. ./run.sh starts a shell in a Docker image that contains the development environment. dub build --build=release generates the HTML and JavaScript assets and puts them into the dist/ directory.

This tutorial is translated from a Japanese post by outlandkarasu, who deserves all the credit for figuring this stuff out.

# Understanding a *nix Shell by Writing One

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A typical *nix shell has a lot of programming-like features, but works quite differently from languages like Python or C++. This can make a lot of shell features — like process management, argument quoting and the export keyword — seem like mysterious voodoo.

But a shell is just a program, so a good way to learn how a shell works is to write one. I’ve written a simple shell that fits in a few hundred lines of commented D source. Here’s a post that walks through how it works and how you could write one yourself.

# Using D Features to Reimplement Inheritance and Polymorphism

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Some months ago I showed how inheritance and polymorphism work in compiled languages by reimplementing them with basic structs and function pointers. I wrote that code in D, but it could be translated directly to plain old C. In this post I’ll show how to take advantage of D’s features to make DIY inheritance a bit more ergonomic to use.

Although I have used these tricks in real code, I’m honestly just writing this because I think it’s neat what D can do, and because it helps explain how high-level features of D can be implemented — using the language itself.