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There’s this one large haskell project, flare-timing, that I regularly wait around 360 seconds to build from scratch. What am I doing with my time? Could splitting the large gap¹ package into smaller packages speed up the compile?

This package, with many reverse dependencies, implements the simpler parts of GAP, the rules for scoring flying competitions. Flights are scored on measures of distance, effort, leading, speed and arrival at goal. Depending on key factors like how many pilots made goal, the weight and allocation of points varies as does the validity of each task in a competition. Penalties and stopped tasks are also dealt with here. All these aspects of scoring are mostly independant of each other so we have an opportunity to break up this package.

Smaller modules compile faster². If I broke up this big package would compile times improve? I hoped so because not every one of its dependants depends on the same set of imports. For instance one exe only deals with scoring effort and another one only deals with scoring leading.

The package exposes one mega-module, Flight.Score³. For the breakup, I created new packages but kept gap around so I could defer updating dependant’s dependencies. For the moment they could stay the same with their imports. Here is how those imports would have looked before the breakup if I had been using the PackageImports extension.

import "flight-gap" Flight.Score (...)

I ended up adding seven gap-* packages⁴ after regrouping modules.

.
├── gap
├── gap-allot
├── gap-effort
├── gap-lead
├── gap-math
├── gap-stop
├── gap-valid
└── gap-weight

The original module imports all the others for re-export.

module Flight.Score
    ( -- explicit exports go here but are not shown
    ) where

import "flight-gap-allot" Flight.Score
import "flight-gap-effort" Flight.Score
import "flight-gap-lead" Flight.Score
import "flight-gap-math" Flight.Score
import "flight-gap-stop" Flight.Score
import "flight-gap-valid" Flight.Score
import "flight-gap-weight" Flight.Score

After making changes so that packages don’t depend on flight-gap but on the finer grained flight-gap-* packages, the build took 315 seconds, an improvement of 12.5%.

To change package dependencies, I swapped packages in the cabal file and added the package name to imports, like with this diff;

-- import qualified Flight.Score as Gap (ReachToggle(..))
-- import Flight.Score (ArrivalFraction(..))
++ import qualified "flight-gap-valid" Flight.Score as Gap (ReachToggle(..))
++ import "flight-gap-allot" Flight.Score (ArrivalFraction(..))

As a finickity last step I could change the exported module names but haven’t got around to this yet.

-- import qualified "flight-gap-valid" Flight.Score as Gap (ReachToggle(..))
-- import "flight-gap-allot" Flight.Score (ArrivalFraction(..))
++ import qualified Flight.Score.Valid as Gap (ReachToggle(..))
++ import Flight.Score.Allot (ArrivalFraction(..))

Experience Report

Putting related modules together and keeping unrelated modules apart by using finer grained packaging was worth it. The code base is better organised. It was easier to reanimate bit-rotten test-suites and have them pass continuous integration when the test surface is smaller. The compile times got a little better.