Mistakes I made writing my first compiler, and what I would do better next time

My most ambitious side project so far has been the Antimony Programming Language. It's a compiler for a programming language I made up about two years ago. It can be used for basic programs and algorithms, but anything beyond bubblesort and the ackermann function wasn't feasible.

Unfortunately, the language never grew out of the "toy" phase, so I more or less abandoned it. Looking back, I definitely learned a lot about compilers, and if I were to write another language (which I will most likely do in the future), there are some things that I would do better next time.

Statements are expressions minus flexibility

The syntax of many C-like programming languages can be broken up into two categories. An Expression is anything that resolves a value.

1 + 2       // 3
5 * 8 + 2   // 42
1 < 2       // true
addOne(1)   // 2 (assuming `addOne` returns the given value plus one)
[1, 2, 3]   // Array containing a 1, a 2 and a 3

A Statement resembles an action or branch in a program.

int foo() {                 // Declare statement
    int bar = 1;            // Assignment statement
    if (bar >= 1) {         // Conditional statement
        printf("%d", bar);  // Function call expressions can be statements too!

Antimony picks up on this concept, and strictly separates statements from expressions. Some modern programming languages (E.g. Rust) got rid of most types of statements by just declaring almost anything an expression. There are just Declaration Statements used for function and variable declaration, and Expression Statements, which can contain any arbitrary expression. This allows us to introduce a lot of nice features that are very hard to implement if we treated statements as actual statements

let foo = if 1 > 2 {
} else {
    for {
        break 2

print(foo)  // 2

Unfortunately, I realized the benefits of extensively using expressions too late.


Don't add types as an afterthought

Multiple backends = exponential headaches

"I won't need an IR" is a lie

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