The Cost of Being Clean: Rethinking Function Purity in Real Systems

There’s a certain elegance to pure functions.

You give them input.

They give you output.

Nothing else happens.

No hidden state.

No side effects.

No surprises.

It feels… clean.

And for a while, it’s tempting to believe that this is how all code should be written.

But the moment you try to build something real, that idea begins to crack.

The Promise of Predictability

Pure functions offer something deeply reassuring.

They behave the same way every time.

No matter where they are called.

No matter when.

This makes them easy to:

• test
• reason about
• combine

They don’t depend on the outside world.

And because of that, they don’t inherit its unpredictability.

In isolation, they feel perfect.

A World That Isn’t Pure

But software doesn’t live in isolation.

It lives in a world of:

• user input
• network requests
• time
• changing state

And none of these things are pure.

A function that fetches data depends on a server.

A function that reads time depends on the clock.

A function that updates the UI changes something outside itself.

These are not edge cases.

They are the system.

The First Friction

So you begin with a simple realization:

Not everything can be pure.

And that realization leads to a question:

If purity is so good—

why can’t we apply it everywhere?

The answer is not that purity is flawed.

It’s that reality is.

Moving Impurity, Not Removing It

The goal shifts.

Instead of trying to eliminate side effects,

you begin to control where they happen.

You keep your core logic pure—

where transformations happen,

where data is shaped,

where meaning is constructed.

And you push impurity outward—

to the edges where the system touches the world.

The boundary becomes intentional.

Not everything is clean—

but the important parts are.

When Clean Code Gets Heavy

There’s another subtle cost.

Purity avoids mutation.

Which means instead of changing things,

you create new ones.

You copy.

You spread.

You rebuild.

And over time, this adds weight.

More memory.

More allocations.

More steps to express something simple.

What was once a direct update

becomes a reconstruction.

The code is safer—

but also heavier.

The Shape of Complexity

Purity simplifies individual functions.

But it can complicate the system as a whole.

Because now:

• data flows through many transformations
• logic is broken into smaller pieces
• behavior is distributed

Nothing is hidden.

But everything is spread out.

And understanding requires following the flow—

not just reading a single function.

When Time Enters the System

There are moments where purity feels especially strained.

Anything involving time.

Anything involving change.

Anything that cannot be repeated in the same way twice.

You can simulate purity—

by passing time as input,

by isolating state.

But that doesn’t remove complexity.

It moves it.

And sometimes, that movement makes things harder to follow.

The Trade You’re Actually Making

Purity is not free.

It trades:

• performance for safety
• simplicity of execution for clarity of reasoning
• direct mutation for controlled transformation

And whether that trade is worth it depends on context.

Not ideology.

Where Purity Feels Right

There are parts of a system where purity fits naturally.

Data transformations.

Calculations.

Derived values.

In these places, purity doesn’t feel like a constraint.

It feels like alignment.

The code reflects the problem.

Cleanly.

Directly.

Where It Starts to Resist

But at the boundaries—

where your system interacts with the outside world—

purity begins to resist.

Because the world is not predictable.

Not repeatable.

Not controlled.

And trying to force purity here

often leads to more abstraction than clarity.

A More Honest Perspective

So the goal is not to make everything pure.

It’s to understand where purity helps—and where it doesn’t.

To use it as a tool, not a rule.

To recognize that some parts of your system benefit from isolation—

while others require interaction.

A Final Reflection

Function purity is not about writing perfect code.

It’s about shaping how complexity flows through your system.

Keeping it contained where it matters.

Allowing it where it’s unavoidable.

It gives you clarity—

but only if you’re willing to accept its cost.

And once you see that,

purity stops being something you chase—

and becomes something you choose.

Deliberately.

With context.

As part of a larger design.