Keeping Compilation Fast

You’re a Haskell programmer, which means you complain about compilation times.

We typically spend a lot of time waiting for GHC to compile code. To some extent, this is unavoidable - GHC does a tremendous amount of work for us, and we only ever ask it to do more. At some point, we shouldn’t be terribly surprised that “doing more work” ends up meaning “taking more time.” However, there are some things we can do to allow GHC to avoid doing more work than necessary. For the most part, these are going to be code organization decisions.

In my experience, the following things are true, and should guide organization:

So let’s talk about some aspects of project organization and how they can affect compile times.

The Project.Types Megamodule

You just start on a new project, and you get directed to the God module - Project.Types. It’s about 4,000 lines long. “All the types are defined in here, it’s great!” However, this is going to cause big problems for your compilation time:

We pretty much can’t take advantage of caching, because GHC doesn’t cache any finer than the module-level. We can’t take advantage of parallelism, as GHC’s parallelism machinery only seems to work at module granularity. Furthermore, we’re tripping this constantly, which is causing GHC to recompile a lot of modules that probably don’t need to be recompiled.


Factor concepts out of your Project.Types module. This will require manually untangling the dependency graph, which can be a little un-fun. It’s probably also a good excuse to learn .hs-boot files for breaking mutual recursion.

There’s a small constant cost to compile a module, so you probably shouldn’t define a module for every single type. Group related types into modules. The sweet spot is probably between 50-200 lines, but that’s a number I just summoned out of the intuitional aether.

This process can be done incrementally. Pick a concept or type from the bottom of your dependency graph, and put it in it’s own module. You’ll need to import that into Project.Types - but do not reexport it! Everywhere that complains, add another import to your new module.

As you factor more and more modules out, eventually you’ll start dropping the dependency on Project.Types. Now, as you edit Project.Types, you won’t have to recompile these modules, and your overall compile-times will improve dramatically. All the types that are pulled out of Project.Types will be cached, so recompiling Project.Types itself will become much faster.

Before too long, you’ll be minimizing the amount of compilation you have to do, and everything will be happy.

Package Splitting

Okay so you think “I know! I’ll make a bunch of packages to separate my logical concerns!” This is probably smart but it comes with some important trade-offs for development velocity and compile-times.


GHCi is pretty picky about loading specific targets, and what you load is going to determine what it will pick up on a reload. You need to ensure that each target has the same default extensions, dependencies, compiler flags, etc. because all source files will be loaded as though they were in a single project. This is a good reason to either use Cabal or hpack common stanzas for all of this information, or to use file-specific stuff and avoid using implicit configuration.

What’s a “load target”? A target is a part of a package, like a library, a specific test-suite, a specific executable, or a sub-library. In a multi-package Cabal or Stack project, load targets can come from different packages.

Another gotcha is that any relative filepaths must resolve based on where you’re going to invoke {stack,cabal} ghci. Suppose you decide you want to split your web app into two packages: database and web, where database has a file it loads for the model definitions, and web has a bunch of files it loads for HTML templating. The Template Haskell file-loading libraries pretty much assume that your paths are relative to the directory containing the .cabal file. When you invoke stack ghci (or cabal repl), it puts your CWD in the directory you launch it, and the relative directories there are probably not going to work.

Once you’ve created that package boundary, it becomes difficult to operate across it. The natural inclination - indeed, the reason why you might break it up - is to allow them to evolve independently. The more they evolve apart, the less easily you can load everything into GHCi.

You can certainly load things into GHCi - in the above example, web depends on database, and so you can do stack ghci web, and it’ll compile database just fine as a library and load web into GHCi. However, you won’t be able to modify a module in database, and hit :reload to perform a minimal recompilation. Instead, you’ll need to kill the GHCi session and reload it from scratch. This takes a lot more time than an incremental recompilation.

Module Parallelism

GHC is pretty good at compiling modules in parallel. It’s also pretty good at compiling packages in parallel.

Unfortunately, it can’t see across the package boundary. Suppose your package hello depends on module Tiny.Little.Module in the package the-world, which also contains about a thousand utility modules and Template Haskell splices and derived Generic instances for data types and type family computations and (etc……). You’d really want to just start compiling hello as soon as Tiny.Little.Module is completely compiled, but you can’t - GHC must compile everything else in the package before it can start on yours.

Breaking up your project into multiple packages can cause overall compile-times to go up significantly in this manner. If you do this, it should ideally be to split out a focused library that will need to change relatively rarely while you iterate on the rest of your codebase. I’d beware of breaking things up until absolutely necessary - a package boundary is a heavy tool to merely separate responsibilities.

At the day job, we have a package graph that looks like this:

         +-> C
A -> B --|
         +-> D

By combining A and B into a single package, we sped up compile times for a complete build of the application by 10%. A clean build of the new AB package was 15% faster to build all told,and incremental builds were improved too.

Package parallelism

The good news is that it is quite easy to cache entire packages, and the common build tools are quite good at compiling packages in parallel. It’s not that big of a deal to depend on lens anymore, largely because of how good sharing and caching has gotten. So certaily don’t be afraid to split out libraries and push them to GitHub or Hackage, but if you’re not willing to GitHub it, then it should probably stay in the main package.

Big Ol Instances Module

Well, you did it. You have a bunch of packages and you don’t want to merge them together. Then you defined a bunch of types in foo, and then defined a type class in bar. bar depends on foo, so you can’t put the instances with the type definitions, and you’re a Good Haskeller so you want to avoid orphan instances, which means you need to put all the instances in the same module.

Except - you know how you had a 4,000 line types module, which was then split-up into dozens of smaller modules? Now you have to import all of those, and you’ve got a big 3,000 class/instance module. All the same problems apply - you’ve got a bottleneck in compilation, and any touch to any type causes this big module to get recompiled, which in turn causes everything that depends on the class to be recompiled.

A solution is to ensure that all your type classes are defined above the types in the module graph. This is easiest to do if you have only a single package. But you may not be able to do that easily, so here’s a solution:

Hidden Orphans

The real problem is that you want to refer to the class and operations without incurring the wrath of the dependency graph. You can do this with orphan instances. Define each instance in it’s own module and import them into the module that defines the class. Don’t expose the orphan modules - you really want to ensure that you don’t run into the practical downsides of orphans while allowing recompilation and caching.

You’ll start with a module like this:

module MyClass where

import Types.Foo
import Types.Bar
import Types.Baz

class C a

instance C Foo
instance C Bar
instance C Baz

where a change to any Types module requires a recompilation of the entirety of the MyClass module.

You’ll create an internal module for the class (and any helpers etc), then a module for each type/instance:

module MyClass.Class where

class C a

module MyClass.Foo where

import MyClass.Class
import Types.Foo

instance C Foo

module MyClass.Bar where

import MyClass.Class
import Types.Bar

instance C Bar

module MyClass.Baz where

import MyClass.Class
import Types.Baz

instance C Baz

module MyClass (module X) where

import MyClass.Class as X
import MyClass.Foo as X
import MyClass.Bar as X
import MyClass.Baz as X

So what happens when we touch Types.Foo? With the old layout, it’d trigger a recompile of MyClass, which would have to start entirely over and recompile everything. With the new layout, it triggers a recompile of MyClass.Foo, which is presumably much smaller. Then, we do need to recompile MyClass, but because all the rest of the modules are untouched, they can be reused and cached, and compiling the entire module is much faster.

This is a bit nasty, but it can break up a module bottleneck quite nicely, and if you’re careful to only use the MyClass interface, you’ll be safe from the dangers of orphan instances.

Some random parting thoughts


(this section was added on 2020-01-30)

Not sure if these tips are legit? Here’s an experience report. Before we implemented these changes at work, a full build of the repository code (56kloc) took 9:55.

Today, a full build of the repository (with GHC flags) is down to 3:40, and we have 70kloc. Much of that comes from the improvements to persistent-template-2.8.0’s performance improvements - about 20%. The rest is module splitting and exposing parallelism.