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.
You just start on a new project, and you get directed to the God module -
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
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
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.
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
Suppose you decide you want to split your web app into two packages:
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
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.
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
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.
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.
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 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:
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
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
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.
persistentquasiquoter takes about 50ms.
persistent-template-2.8.0, which dramatically speeds up compile-times by generating less code)
stack build --fast --file-watch --ghc-options "-j4 +RTS -A128m -n2m -RTS"
These flags give GHC 4 threads to work with (more didn’t help on my 8 core computer), and
-A128m gives it more memory before it does GC.
(EDIT: this post used to recommend
-qg which turns off the parallel garbage collector. @MaxTagher did the brave work of actually verifying this, and it was indeed slowing things down! removing this flag dropped a full build from 3:28 to 3:03, a huge improvement.)
Thanks to /u/dukerutledge for pointing out
-n2m, which I don’t understand but helped!
ghcifriendly as much as possible.
:reloadis the fastest way to test stuff out usually, and REPL-friendly code is test-friendly too!
(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.