Writing a Simple DSL Compiler with Delphi [7. AST Compiler]

This article provides a description of an AST compiler used for my toy language project. If you are new to this series, I would recommend to start reading with this post. At least you should read the previous post, Intermezzo, as it explains some parts of the compiler that I won't touch here.

Please note that this article describes an initial implementation of the compiler. If you want to browse the code while reading the article, make sure that you have switched to branch dsl_v1.

In my toy compiler framework, a compiler (or codegen as it is called internally), is a piece of code that implements the ISimpleDSLCodegen interface. This interface exposes only one function, Generate, which takes an abstract syntax tree and converts it into an object implementing an ISimpleDSLProgram interface which allows you to call any function in a compiled program by name.

type

  TParameters = TArray;

  TFunctionCall = reference to function (const parameters: TParameters): integer;

  ISimpleDSLProgram = interface ['{2B93BEE7-EF20-41F4-B599-4C28131D6655}']

    function  Call(const functionName: string; const params: TParameters;       var return: integer): boolean;

   end;

  ISimpleDSLCodegen = interface ['{C359C174-E324-4709-86EF-EE61AFE3B1FD}']

    function Generate(const ast: ISimpleDSLAST;      
      var runnable: ISimpleDSLProgram): boolean;

  end;

Writing a Simple DSL Compiler with Delphi [Intermezzo]

When I was preparing an article about the compiler part of my toy language project, I found out that the concept of wrapping a whole program into a bunch of anonymous functions (what the compiler does) is exceedingly hard to explain. I had therefore prepare a simplified version of the compiler, written for a very simplified language ... and then I couldn't stop and I added an AST, parser, tokenizer, interpreter and all shebang.

The result of all that is a program introduction.dpr, a self-contained console program which contains a complete (almost trivial) language together with the full documentation, written in a Literate Programming style. Simply put - you can read it from top to bottom like a story.

As an intermezzo and to simplify my explanation of the compiler, I'm posting the whole program here, reformatted as a blog post.

Writing a Simple DSL Compiler with Delphi [6. AST Dumper]

This article provides a description of a testing tool used for my toy language project. If you are new to this series, I would recommend to start reading with this post. 

Please note that this article describes an initial implementation of the parser. If you want to browse the code while reading the article, make sure that you have switched to branch dsl_v1.

Now that we have a working tokenizer and parser outputting an AST, we can start working on the compiler. Still, it would be great if we can verify whether the parser output (the AST) makes any sense. In other words, we need unit tests.

Writing unit tests for a tree structure is, however, a very tedious operation. In the next post I'll show a test for a tree with only five nodes and it will already be a process one would rather skip. Luckily, we can do something more fun - we can write a code that recreate the original program from an AST.

Writing a Simple DSL Compiler with Delphi [4. Parser]

This article provides a description of a parser used in my toy language project. If you are new to this series, I would recommend to start reading with this post.

Please note that this article describes an initial implementation of the parser. If you want to browse the code while reading the article, make sure that you have switched to branch dsl_v1.

After a short break I'm returning to my "toy compiler" series. This time I'll describe the working of the parser - the part of the code which reads the input (usually in a tokenized form) and generates internal representation of the program (in my case an abstract syntax tree - AST).

The goal of my project was to study the compilation step and parser was just a necessary evil that I had to deal with. That's why it is written in a pretty primitive manner, without using any improvements like a Pratt parser.

Writing a Simple DSL Compiler with Delphi [3. Tokenizer]

This article provides a description of a tokenizer used in my toy language project. If you are new to this series, I would recommend to start reading with this post.

Please note that this article describes an initial implementation of the tokenizer. If you want to browse the code while reading the article, make sure that you have switched to branch dsl_v1.

With this article I'm moving into the gritty part of the project - the code which reads the source code and turns it into a beautiful abstract syntax tree. In other words, I'll be talking about the parser.

I must admit that I spent as little time on the parser as I could. After all, my main goal was to convert AST into an executable code, not to parse input. Still, one cannot write a compiler without writing a parser, so ... here I am.

Writing a Simple DSL Compiler with Delphi [2. Abstract Syntax Tree]

This article provides a description of an abstract syntax tree used to represent "The Language". If you are new to this series, I would recommend to start reading with this post. 

Please note that this article describes an initial implementation of the AST. If you want to browse the code while reading the article, make sure that you have switched to branch dsl_v1.

An abstract syntax tree (AST) is, simply put, a symbolic representation of the program in a form of a tree.

While the textual representation of a program is great for us, humans, computers have hard time dealing with it. Because of that, a special part of any interpreter/compiler, called parser, reads the input and converts it into a computer-readable format - AST. This tree can then be used for multiple purposes. We can, for example, feed to into an interpreter which will then run the program for us, or we can feed it into a compiler to generate an executable program or cross-compiler to generate an equivalent program in a different programming language.

Writing a Simple DSL Compiler with Delphi [1. The Language]

Part 1: The Language

This article provides an informal definition of a simple language (a.k.a. "The Language") I am writing a compiler for. If you are new to this series, I would recommend to start reading with this post.

Let's start with a simple example which calculates an i-th Fibonacci number.

 fib(i) {
   if i < 3 {
     return 1
   } else {
     return fib(i-2) + fib(i-1)
   }
 }

The code is quite simple. First two numbers in a Fibonacci sequence are 1 and every other Fibonacci number is a sum of previous two in the sequence.

Writing a Simple DSL Compiler with Delphi [0. Introduction]

Part 0: Introduction

Some time ago I was listening to the Hanselminutes podcast where a guy described how he wrote a simple interpreter in Go language (YOU should write an interpreter with Thorsten Ball). I was not really interested in writing an interpreter - I did that a long time ago - but a thought crossed my mind. I asked myself whether I can do something better - write a compiler. (Or, rather, a kinda-compiler. You'll see.)

What I wanted to do is to take a small language, parse it, generate an abstract syntax tree, and then convert this into one large anonymous function calling other anonymous functions calling other anonymous functions and so on and so on. It is hard to explain, so let me try using a very simple example.