blog-static/content/blog/04_compiler_improvements.md

title	date	draft	tags
Compiling a Functional Language Using C++, Part 4 - Small Improvements	2019-08-06T14:26:38-07:00	true	C and C++ Functional Languages Compilers

We've done quite a big push in the previous post. We defined type rules for our language, implemented unification, and then implemented unification to enforce these rules for our program. The post was pretty long, and even then we weren't able to fit quite everything into it.

For instance, we threw 0 whenever an error occured. This gives us no indication of what actually went wrong. We should probably define an exception class, one that can contain information about the error, and report it to the user.

Also, when there's no error, our compiler doesn't really tell us anything at all about the code besides the number of definitions. We probably want to see the types of these definitions, or at least some intermediate information. At the very least, we want to have the ability to see this information.

Finally, we have no build system. We are creating more and more source files, and so far (unless you've taken initiative), we've been compiling them by hand. We want to only compile source files that have changed, and we want to have a standard definition of how to build our program.

Setting up CMake

This would be extremely easy if not for Flex and Bison. We start with the usual: {{< codelines "CMake" "compiler/04/CMakeLists.txt" 1 2 >}}

Next, we want to set up Flex and Bison. CMake provides two commands for this: {{< codelines "CMake" "compiler/04/CMakeLists.txt" 4 5 >}}

We now have access to commands that allow us to tell CMake about our parser and tokenizer (or scanner). We use them as follows: {{< codelines "CMake" "compiler/04/CMakeLists.txt" 6 12 >}}

We also want CMake to know that the scanner needs to parser's header file in order to compile. We add this dependency: {{< codelines "CMake" "compiler/04/CMakeLists.txt" 13 13 >}}

Finally, we add our source code to a CMake target. We use the BISON_parser_OUTPUTS and FLEX_scanner_OUTPUTS to pass in the source files generated by Flex and Bison. {{< codelines "CMake" "compiler/04/CMakeLists.txt" 15 22 >}}

Almost there! parser.cpp will be generated in the build directory during an out-of-source build, and so will parser.hpp. When building, parser.cpp will try to look for ast.hpp, and main.cpp will look for parser.hpp. We want them to be able to find each other, so we add both the source directory and the build (binary) directory to the list of includes directories:

{{< codelines "CMake" "compiler/04/CMakeLists.txt" 23 24 >}}

That's it for CMake! Let's try our build:

cmake -S . -B build
cd build && make -j8

We get an executable called compiler. Excellent! Here's the whole file: {{< codeblock "CMake" "compiler/04/CMakeLists.txt" >}}