Things I Realized While Implementing a Language.

by Tim Anema

I just wanted to list a couple things that may not be apparent if you went into building a programming language. These are things that made me 🤔.

The VM does’t care about variable names.

Humans need variable names, machines don’t. The VM and especially assembly code doesn’t need variable names, they are just positional values operated on at specific locations in the stack or specific registers. A lot of naive implementations of languages will start off keeping track of variable names and this will overcomplicate their design. Going further, the names are kept around but only for humans to debug.

Garbage collection cares about data that escapes it’s declaration scope

We so often talk about garbage collection because it can be a point of slow down however that talk usually lacks a nuance about what is collected and when. It wasn’t until writing my own GC that I realized, if a value is allocated on the stack but never leaves it’s scope, it is simply removed from the stack at the end of the scope. Not a big deal. GC only cares if that value was allocated and then is captured in another scope like a closure. Now it has to keep track of that value and how much it is used so that it can still be available when needed and discarded when no longer needed.

You’re really writing two languages

When you write a new higher level language you also have to write your intermediate representation bytecode which is another simpler language. This is often referred to as the language’s frontend and backend. They can largely be unit tested separately, because of this. The frontend is just a parser that converts high language to a low language API commands. The backend responds to each command and updates its stack, gc, etc based on the action.

Const folding

Const folding is a really cool feature of languages. There are many times where you will define config varaibles like this:

local speed = 1000
local accel = speed * 2

But by the time the variable accel makes it to the backend, it has already been evaluated to 2000 as it doesn’t make sense to run extra code if it can be consistently reasoned that the value starts as such. This is called const folding. It looks for const values and will try to optimize any evaluation that we can ensure wont change during evaluation.

To be continued

I will probably add to this in the future but that is it for now.