Rachit Nigam

Dear Sir, You Have Built a Compiler

January 11, 2022

Dear Sir,

I am afraid to inform you that you have built a compiler. I know you wanted a “simple prototype” that would just add that one feature to your programming model. You said that “SSA is an overkill” and “it’s just way too much infrastructure to maintain for a simple task” and yet, six months later, you have pile of string mangling scripts that do not work—breaking every time a user input slightly deviates from things you’ve seen before.

Surely, switching to the unstable abstract syntax tree (AST) library provided by the compiler will be the end of your woes; at least that way, someone else maintains a real parser and provides at least a semblance of sane input for you to transform. But wait, the AST is massive. “Do I really have to handle all 500 different AST nodes?” you ask yourself? Surely not. Your users aren’t crazy; they don’t use all the weird features of this language. So you march on, and handle the 50 AST nodes that matter, certain, that this will be the last of what you need to do to maintain this pile of hacks.

Ah, but wait! Inevitably, someone wanted to nest a for loop inside a switch statement, a struct definition within that loop, and a conditional in expression position. Tired, you patch in support for each feature, distracting from the crucial features you should be working on and shipping. One of your brilliant team member suggests a pre-processing stage: de-nest all definitions, hoist all expressions, flatten out all control, and that way, you only have to handle 50 AST nodes because you will know, yes know, that the program cannot have any other shape.

Except once that engineer leave, who will know what assumptions you encoded? Those littered asserts? The inscrutable “unreachable code” errors? Who will know, how you simplified your AST? So you rolled out your own AST library, so that you may compile, nay, transpile your code and expose your assumptions in your data structures. Certain, of course, that because you’re transpiling JavaScript to JavaScript, it is going to way easier than what real compilers do. What glorious engineering, you say to yourself.

In the last leg of your journey to avoid building a compiler, your manager tells you that your code should run on older machines, which only support version 0.8. Version 0.8, of course, does not support the brilliant type-level encodings your transpiler generates to implement your feature. Not my problem, your manager says. So you write some code that simplifies your transpiled code further, making it use only features present in version 0.8. Done at last, you say to yourself, without having to build a compiler.

A parser, an intermediate representation, transformation passes, and a code generator. Dear Sir, you have built a compiler.

Addressed to,
Those who did not want to build a compiler