This blog post is intended to answer two very frequest questions about stack: how is it different from Cabal? And: Why was it developed as a separate project instead of being worked on with Cabal?
Before we delve into the details, let’s first deconstruct the premises of the questions. There are really three things that people talk about when they say “Cabal”:
.cabalfiles) and specification for a “common architecture for building applications and tools”, aka Cabal-the-spec,
cabal-install, aka Cabal-the-tool, which is a command-line tool that uses Cabal-the-library.
Stack complies with Cabal-the-spec, both in the sense that it
.cabal files in their entirety and behaves in a
way that complies with the spec (insofar as that is relevant since
the spec hasn’t seen any updates in recent years). In fact it was
easy for Stack to do, because just like Cabal-the-tool, it is
implemented using Cabal-the-library. Therefore, a first answer to
the questions at hand is that stack is Cabal: it is 100%
compatible with the existing Cabal file format for specifying
package metadata, supports exactly the same package build harnesses
and is implemented on top of the same reference implementation of
the spec as
cabal-install, which is just one tool
among others using Cabal-the-library.
and stack are separate tools that both share the same framework. A
successful framework at that: Haskell’s ecosystem would not be
where it is today without Cabal, which way back in 2004, for the
first time in the long history of Haskell made it possible to
easily reuse code across projects by standardizing the way packages
are built and used by the compiler.
Stack is different in that it is a from-the-ground-up rethink of Cabal-the-tool. So the real questions are: why was a new tool necessary, and why now? We’ll tackle these questions step-by-step in the remainder of this post:
Stack was started because the Haskell ecosystem has a tooling problem. Like any number of other factors, this tooling problem is limiting the growth of the ecosystem and of the community around it. Fixing this tooling problem was born out of a systematic effort of growth hacking: identify the bottlenecks that hamper growth and remove them one by one.
The fact that Haskell has a tooling problem is not a rumour, nor
is it a fringe belief of disgruntled developers. In an effort to
collect the data necessary to identifying the bottlenecks in the
growth of the community, FP Complete conducted a wide
survey of the entire community on behalf of the Commercial
Haskell SIG. The results are in and the 1,200+ respondents are
unequivocal: package management with
the single worst aspect of using Haskell. Week after week, Reddit
and mailing list posts pop up regarding basic package installation
cabal-install. Clearly there is a
problem, no matter whether seasoned users understand their tools
well, know how to use it exactly right and how to back out
gracefully from tricky situations. For every battle hardened power
user, there are 10 enthusiasts willing to give the language a try,
if only simple things were simple.
Of a package building and management tool, users expect, out-of-the-box (that means, by default!):
In fact these are the very same desirable properties that Johan Tibell identified in 2012 and which the data supports today. If our tooling does not support them, this is a problem.
Stack is an attempt to fix this problem – oddly enough, by
building in at its core much of the same principles that underlie
how power users utilize
The key to stack’s success is to start from common workflows,
choosing the right defaults to support them, and making those
One of the fundamental problems that users have with package management systems is that building and installing a package today might not work tomorrow. Building and installing on my system might not work on your system. Despite typing exactly the same commands. Despite using the exact same package metadata. Despite using the exact same version of the source code. The fundamental problem is: lack of reproducibility. Stack strives hard to make the results of every single command reproducible, because that is the right default. Said another way, stack applies to package management the same old recipe that made the success of functional programming: manage complexity by making the output of all actions proper functions of their inputs. State explicitly what your inputs are. Gain the confidence that the outputs that you see today are the outputs that you see tomorrow. Reproducibility is the key to understandability.
In the cabal workflow, running
cabal install is
necessary to get your dependencies. It’s also a black box which
depends on three pieces of global, mutable, implicit state: the
compiler and versions of system libraries on your system, the Cabal
packages installed in GHC’s package database, and the package
metadata du jour downloaded from Hackage (via cabal update).
cabal install at different times can lead to
wildly different install plans, without giving any good reason to
the user. The interaction with the installed package set is
non-obvious, and arbitrary decisions made by the dependency solver
can lead to broken package databases. Due to lack of isolation
between different invocations of
cabal install for
different projects, calling
cabal install the first
time can affect whether
cabal install will work the
second time. For this reason, power users use the
freeze feature to pin down exactly the version of every
dependency, so that every invocation of
always comes up with the same build plan. Power users also build in
so-called “sandboxes”, in order to isolate the actions of calling
cabal install for building the one project from the
actions of calling
cabal install for building this
In stack, all versions of all dependencies are explicit and
determined completely in a
stack.yaml file. Given the
stack.yaml and OS, stack build should always run
the exact same build plan. This does wonders for avoiding
accidentally breaking the package database, having reproducible
behavior across your team, and producing reliable and trustworthy
build artifacts. It also makes it trivial for stack to have a
user-friendly UI of just installing dependencies when necessary,
since future invocations don’t have to guess what the build plan of
previous invocations was. The build plan is always obvious and
manifest. Unlike cabal sandboxes, isolation in stack is complete:
packages built against different versions of dependencies never
interfere, because stack transparently installs packages in
separate databases (but is smart enough to reuse databases when it
is always safe to do, hence keeping build times low).
Note that this doesn’t mean users have to painstakingly write out all package versions longhand. Stack supports naming package snapshots as shorthand for specifying sets of package versions that are known to work well together.
Other key design principles are portability (work consistently and have a consistent UI across all platforms), and very short ramp-up phase. It should be easy for a new user with little knowledge of Haskell to write “hello world” in Haskell, package it up and publish it with just a few lines of configuration or none at all. Learning a new programming language is challenge enough that learning a new package specification language is quite unnecessary. These principles are in contrast with those of platform specific and extremely general solutions such a Nix.
Modularity (do one thing and do it well), security (don’t trust stuff pulled from Internet unless you have a reason to) and UI consistency are also principles fundamental to the design, and a key strategies to keeping the bug count low. But more on that another time.
These have informed the following “nice to have” features
The technologies underpinning these features include:
These technologies have enabled swift development of stack
without reinventing the wheel and have helped keep the
implementation stack simple and accessible. With the benefit of a
clean slate to start from, we believe stack to be very hackable and
easy to contribute to. These are also technologies that
cabal-install did not have the benefit of being able
to use when it was first conceived some years ago.
cabal-install, stack and other tools
Stack is but one tool for managing packages on your system and
building projects. Stack was designed specifically to interoperate
with the existing frameworks for package management and package
building, so that all your existing packages work as-is with stack,
but with the added benefit of a modern, predictable design. Because
stack is just Cabal under the hood, other tools such as
Halcyon for deployment and Nix are good
fits complement stack nicely, or indeed
for those who prefer to work with a UI that they know well. We have
already heard reports of users combining these tools to good
effect. And remember: stack packages are
packages are super-new-fangled-cabal-tool packages. You can write
the exact same packages in stack or in another tool, using curated
package sets if you like, tight version bounds à la PVP if you
like, none or anything at all. stack likes to make common usage
easy but is otherwise very much policy agnostic.
Stack is a contributor friendly project, with already 18 contributors to the code in its very short existence, several times more bug reporters and documentation writers, and counting! Help make stack a better tool that suits your needs by filing bug reports and feature requests, improving the documentation and contributing new code. Above all, use stack, tell your friends about it. We hope stack will eliminate a documented bottleneck to community growth. And turn Haskell into a more productive language accessible to many more users.