Building a Typechecker from scratch
Course description
Untyped programs are often prone to errors, runtime exceptions, and can make debugging much harder. That’s why many production languages implement a static typechecker — an extra module, which is aimed to increase programs safety and make development simpler.
Type checking or type inference? What is Type theory and Type judgements? Is my language weakly or strongly typed? And how am I actually going to implement a typechecker?
There are so many questions when it comes to implementing this module. If you’ve been asking those questions in implementing your programming language, or just want to understand how typeckechers work under the hood, on a hands-on practical implementation, this course is for you.
Often related books on Type theory and type judgements go to theoretical aspects viewing types as mathematical sets, not explaining how actually to build a practical typechecker. I believe we should be able to build and understand a typechecker for a full programming language, end-to-end, in 2-4 hours — with a content going straight to the point, showed in live coding sessions as pair-programming and described in a comprehensible way.
In the Building a Typechecker from scratch class we focus specifically on a static typechecker, and build a similar to TypeScript, Java, etc. We slightly touch Type theory and already since the first lecture go into the practical implementation.
Implementing a typechecker would also increase your engineering level, as it touches several aspects of data structures and algorithms.
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Prerequisites
An optional prerequisite for this class is the Building an Interpreter from scratch (aka Essentials of Interpretation) course, where we build an AST-interpreter for a full programming language. Unless you already have understanding of how programming languages work at this level, i.e. what eval, a closure, a scope chain, environments, and other constructs are — it is recommended to take the interpreters class as a prerequisite.
Who this class is for?
This class is for any curious engineer, who would like to gain skills of building complex systems (and building a typechecker for a programming language is an advanced engineering task!), and obtain a transferable knowledge for building such systems.
If you are interested specifically in compilers, PL and Type theory, and want to build a typechecker module for your programming language, this class is also for you.
What is used for implementation?
Sometimes people think that to build a typechecker you certainly need to use some programming language which is itself “very related to types and type theory” (such as OCaml of other functional programming languages). But actually no! You can build a fully working typechecker in plain JavaScript, and cover all the aspects of the Type systems. And this is exactly what we use for implementation.
JavaScript, being the most popular programming language, should be accessible for many engineers, and also our goal is to provide a simple and concise implementation, not focusing on specifics of a host language. So the code should be portable to any language of your taste and choice: TypeScript, Rust, OCaml, C++, Python, etc.
Note: we want our students to actually follow, understand and implement every detail of the Typechecker themselves, instead of just copy-pasting from final solution. Even though the full source code for the typechecker is presented in the video lectures, the code repository for the project contains <em>/* Implement here */</em> assignments, which students have to solve.
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All Course Lessons (20)
| # | Lesson Title | Duration | Access |
|---|---|---|---|
| 1 | Introduction to Type theory and checking Demo | 19:39 | |
| 2 | Typing Numbers and Strings | Testing | 04:18 | |
| 3 | Math binary operations | String concat | 06:46 | |
| 4 | Variables and Typing Environment, Р“ | 08:17 | |
| 5 | Blocks and Local scope | 06:55 | |
| 6 | Parsing: S-expression to AST | 07:31 | |
| 7 | Control flow: If and While expressions | 07:25 | |
| 8 | User-defined functions | Local environments | 09:16 | |
| 9 | Function calls | Built-in functions | 04:42 | |
| 10 | Closures | Recursive calls | 05:12 | |
| 11 | Lambda functions and IILE | Syntactic sugar | 04:30 | |
| 12 | Declaring new types | Type aliases | 03:54 | |
| 13 | OOP | Classes | 06:40 | |
| 14 | OOP | Instances | 03:51 | |
| 15 | Super calls | Inheritance | 02:49 | |
| 16 | Union type | 06:02 | |
| 17 | Union | Type narrowing | 07:24 | |
| 18 | Generics | Function declarations | 06:51 | |
| 19 | Generics | Function calls | 07:09 | |
| 20 | Final executable | 07:04 |
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