Awesome

Pier: Yet another Haskell build system.

Pier is a command-line tool for building Haskell projects. (Yes, another one.)

Pier is similar in purpose to Stack; it uses *.cabal files for package configuration, and uses Stackage for consistent sets of package dependencies. However, Pier attempts to address some of Stack's limitations by exploring a different approach:

• Pier invokes tools such as ghc directly, implementing the fine-grained Haskell build logic from (nearly) scratch. In contrast, Stack relies on a separate framework to implement most of its build steps (i.e., Cabal/Distribution.Simple), giving it mostly coarse control over the build.
• Pier layers its Haskell-specific logic on top of a general-purpose library for hermetic, parallel builds and dependency tracking. That library is itself implemented using Shake, and motivated by tools such as Nix and Bazel. In contrast, Stack's build and dependency logic is more specific to Haskell projects.

(Interestingly, Stack originally did depend on Shake. The project stopped using it early on, in part due to added complexity from the extra layer of Cabal build logic. For more information, see write-ups by authors of Stack and Shake.)

For examples of project configuration, see the sample project, or alternately pier itself.

Status

Pier is still experimental. It has been tested on small projects, but not yet used in anger.

Pier is already able to build most the packages in Stackage (specifically, 93% of the more than 2300 packages in lts-12.8). There is a list of open issues to increase Pier's coverage. (Notably, packages with Custom Setup.hs scripts are not supported.)

Contents

• Installation
• Project Configuration
• Using pier
• Build Outputs
• Frequently Asked Questions

Installation

First clone this repository, and then build and install the pier executable using stack (version 1.6 or newer):

git clone https://github.com/judah/pier.git
cd pier
stack install

Add ~/.local/bin to your $PATH in order to start using pier. For example, try:

cd example
pier build

Project Configuration

A pier.yaml file specifies the configuration of a project. For example:

resolver: lts-10.3
packages:
  - '.'
  - 'foo'
  - 'path/to/bar'

resolver

The resolver specifies a set of package versions (as well as a version of GHC), using Stackage. It can be either an LTS or nightly version. For example:

resolver: lts-10.3

resolver: nightly-2018-02-10

packages

The packages section lists paths to local directories containing Cabal packages (i.e., *.cabal and associated source files). For example:

packages:
  - '.'
  - 'foo'
  - 'path/to/bar'

extra-deps

An extra-deps section may be used to add new versions of packages from Hackage that are not in the resolver, or to override existing versions. For example:

extra-deps:
  - text-1.2.3.4
  - shake-0.15

system-ghc

By default, pier downloads and installs its own, local copy of GHC from github.com/stackage. To override this behavior and use a GHC that's already installed on the system, set:

system-ghc: true

This setting will make pier look in the $PATH for a binary named ghc-VERSION, where VERSION is the version specified in the resolver (for example: ghc-8.2.2).

ghc-options

A list of command-line flags to pass to GHC when compiling packages. For example:

ghc-options: [-O2, -Wall]

or:

ghc-options:
- -O2
- -Wall

Using pier

For general comnmand-line usage, pass the --help flag:

pier --help
pier build --help
pier run --help
# etc.

Common Options

pier build

pier build {TARGETS} builds one or more Haskell libraries and/or binaries from the project, as well as their dependencies. There are a few different ways to specify the targets:

pier run

pier run {TARGET} {ARGUMENTS} builds the given executable target, and then runs it with the given command-line arguments. {TARGET} should be a specific executable; either:

For example, pier run foo is equivalent to pier run foo:exe:foo. Note that this behavior differs from Stack, which is less explicit: stack exec foo may run a binary named foo from any previously built package.

By default, the executable will run in the same directory where pier.yaml is located. To run in a temporary, hermetic directory, use pier run --sandbox.

In case of ambiguity, -- can be used to separate arguments of pier from arguments of the target.

pier test

pier test {TARGETS} builds and tests one or more Cabal test-suites from the project and/or its dependencies. There are a few different ways to specify the targets:

pier which

pier which {TARGET} builds the given executable target and then prints its location. See the documentation of pier run for details on the syntax of {TARGET}.

pier clean

pier clean marks some metadata in the Shake database as "dirty", so that it will be recreated on the next build. This command may be required if you build a new version of pier, but should be unnecessary otherwise.

pier clean-all

pier clean-all completely deletes all build outputs (other than downloaded files, as described here), so that future builds will start from scratch. Note that this command will require Pier to reinstall a local copy of GHC unless system-ghc: true is set.

pier setup

pier setup downloads and configures the base build prerequisites. This includes:

• Downloading and preparing a local installation of GHC
• Downloading and parsing the Stackage build plan
• Parsing the local pier.yaml and *.cabal files.

In general, it should not be necessary to run pier setup explicitly, since those steps are also performed automatically for other commands like build, run and test.

Verbosity

The -V command-line flag will make Pier more verbose. It may be chained to increase verbosity (for example: -VV, -V -V, -VVV).

The verbose output includes (but is not necessarily limited to):

• -V: Upon failure of an invocation of a command-line process (for example, ghc), display the full invocation of that command including all command-line flags and build inputs.
• -VV: Display the full invocation of every command before running it.
• -VVV: Also display internal Shake debug information.

Build Outputs

pier saves most output files in a folder called _pier/, located in the same directory as pier.yaml. The only exception is downloaded files (for example, package tarballs for dependencies), which are saved under $HOME/.pier so that they may be reused between different projects on the same machine.

Each build command (for example, a single invocation of ghc or ghc-pkg) runs separately in a temporary directory with a limited, explicit set of input files. This approach is inspired by the Bazel project, which sandboxes each command in order to get reliable, deterministic builds. Note though that Pier does not currently provide the same strict guarantees as Bazel. Instead, it uses file organization and marking outputs as read-only to catch a subset of potential bugs in the build logic.

The outputs of each command are saved into a distinct directory of the form:

`_pier/artifact/{HASH}`

where the {HASH} is a unique string depending on the command's command-line arguments and input dependencies. This file organization is similar to Nix, though Pier aims for much more fine-grained build steps than a standard Nix package.

Build outputs are also mirrored into a shared cache, located by default at ~/.pier/artifact/{HASH}. Files are hard-linked between there and the local _pier. This enables sharing work between multiple projects. To disable this behavior, use the command-line flag --no-shared-cache. To change the location, use --shared-cache-path, or set the PIER_SHARED_CACHE environmental variable.

If necessary, pier clean-all will delete the _pier folder (and thus wipe out the entire build). That folder can also be deleted manually with chmod -R u+w _pier && rm -rf _pier. (Files and folders in _pier are marked as read-only.)

Frequently Asked Questions

How much of Cabal/Stack does this project re-use?

pier implements nearly all build logic itself, including: configuration, dependency tracking, and invocation of command-line programs such as ghc and ghc-pkg. It uses Cabal/Hackage/Stackage in the follow ways:

• Downloads Stackage's build plans from github.com/fpco, and uses them to get the version numbers for the packages in that plan and for GHC.
• Downloads GHC releases from github.com/commercialhaskell, getting the exact download location from a file hosted by github.com/stackage.
• Downloads individual Haskell packages directly from Hackage.
• Uses the Cabal library to parse the .cabal file format for each package.

In particular, it does not:

• Call the stack executable or depend on the stack library
• Call the cabal binary
• Import Distribution.Simple{.*} from the Cabal library

I heard you like pier, so I built pier with pier.

Building pier with pier is OK, I guess:

pier build pier

But what about using that pier to build pier? We'll just need to distinguish Shake's metadata between the two invocations:

$ pier -- run pier build pier \
        --shake-arg=--metadata=temp-metadata
Build completed in 0:10m

Build completed in 0:10m

The inner run of pier build only takes about 10 seconds on my laptop, because it reuses all of the build outputs that were created by the outer call to pier (and were stored under _pier/artifacts). It spends its time parsing package metadata, computing dependencies, and (re)creating all the build commands in the dependency tree.