Joachim Breitner

Faster Winter 4: Export lists

Published 2019-11-21 in sections English, Haskell.

(This is the forth optimization presented in the “faster winter” series, please see that post for background information.)

This is on a funny one: You can speed up your code by adding export lists to your modules!

Why is that so?

Without an export, the compiler has to assume that every top-level function can possibly called from the outside, even functions that you think of as “internal”. If you have a function that you do not export, like instr, step_work and step after my change, the compiler can see all the places the function is called. If the function is only called in one place, it may inline it (copy its definition into where it is called), and simplify the code around the edges. And even if it does not inline the function, it might learn something about how the functions are used, and optimize them based on that (e.g. based on Demand Analysis).

Improvement: Allocations: -22.59% Memory: +0.00% Time: -11.79% (Commit bbe8af7...6f2ba09)

Besides being a performance win, an explicit export list is also very helpful to those who want to read or edit your code: they can refactor with greater ease while maintaining the external interface of the module.

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