minirepo

A monorepo template, using Nix, direnv and Bazel.

This repository is designed to act as a skeleton for those wishing to use Bazel to build code in a monorepository. It uses Nix to complement Bazel's hermetic builds with a hermetic system environment and direnv to make this implicitly available when users cd into the repository's directory. At present this template supports the following:

• A set of Nixpkgs pins designed to provide flexible control over which versions of which packages and toolchains are available.

• A Haskell toolchain designed for building fully-statically-linked binaries, that is, binaries with no dynamic linking. Such binaries have minimal runtime dependencies and are thus easy to deploy, whether standalone or in bare-minimum Docker containers (as supported by e.g. rules_docker).

• A Docker toolchain capable of building Docker containers with binaries and outputs from this repository (such as the aforementioned static Haskell binaries).

Layout

• nix contains Nix code for pinning package sets and tools used to make working in this repository reproducible.

• direnv contains Bash scripts for having the aforementioned Nix tools be added to your PATH whenever you are working in this repository. These scripts also take care of caching to minimise the performance hits when cding in and out of this repository. .envrc in the repository root is the file which direnv will look for when cding into this repository and is a symlink into the direnv directory, which we use to keep things neat.

• WORKSPACE defines the Bazel workspace and includes rules for building Haskell code, Docker containers, etc. It pulls in definitions from the bazel directory.

• example-service defines an example Haskell service, with a library ifc (short for "interface") and a binary impl (short for "implementation"). Its BUILD.bazel file defines targets for these two things as well as some other test cases (e.g. REPLs, Docker images, Haddock documentation, etc.).

• hie.yaml and .hie-bios are files that use the aforementioned Bazel rules to configure GHCIDE to work with the Haskell projects in this repository.

Cookbook

Bazel

Haskell

• Building a fully-statically-linked Haskell binary

$ bazel run //example-service:impl
INFO: Analyzed target //example-service:impl (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //example-service:impl up-to-date:
  bazel-bin/example-service/impl
INFO: Elapsed time: 0.182s, Critical Path: 0.01s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
42
Hello, world, from a library!
DefaultCompression
DefaultCompression
No connection string given, no PostgreSQL testing performed

$ ldd bazel-bin/example-service/impl
$       not a dynamic executable

• Passing a connection string to test PostgreSQL functionality (and thus more complex C library interactions)

$ PG_CONNECTION_STRING="postgresql://user:pass@localhost" bazel run //example-service:impl
INFO: Analyzed target //example-service:impl (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //example-service:impl up-to-date:
  bazel-bin/example-service/impl
INFO: Elapsed time: 0.158s, Critical Path: 0.01s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
42
Hello, world, from a library!
DefaultCompression
DefaultCompression
Testing PostgreSQL
2

• Building a Docker container containing a fully-statically-linked Haskell binary

$ bazel run //example-service:impl-image
INFO: Analyzed target //example-service:impl-image (47 packages loaded, 6571 targets configured).
INFO: Found 1 target...
Target //example-service:impl-image up-to-date:
  bazel-bin/example-service/impl-image-layer.tar
INFO: Elapsed time: 9.680s, Critical Path: 0.61s
INFO: 10 processes: 10 linux-sandbox.
INFO: Build completed successfully, 15 total actions
INFO: Build completed successfully, 15 total actions
6b79aa9adb28: Loading layer [==================================================>]  45.74MB/45.74MB
Loaded image ID: sha256:57977ee8dc73dc9d44805c8f209305c76a7d37687d08fd4a8adcd2c4d6b0ac7f
Tagging 57977ee8dc73dc9d44805c8f209305c76a7d37687d08fd4a8adcd2c4d6b0ac7f as bazel/example-service:impl-image
shell-init: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory
42
Hello, world, from a library!
DefaultCompression
DefaultCompression
No connection string given, no PostgreSQL testing performed

• Loading a REPL for a Haskell binary

$ bazel run //example-service:impl@repl
INFO: Analyzed target //example-service:impl@repl (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //example-service:impl@repl up-to-date:
  bazel-bin/example-service/impl@repl@repl
INFO: Elapsed time: 0.116s, Critical Path: 0.01s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
GHCi, version 8.6.5: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( example-service/impl/Main.hs, interpreted )
Ok, one module loaded.
Ok, one module loaded.
Loaded GHCi configuration from .../bazel-out/k8-fastbuild/bin/example-service/ghci-repl-script-impl@repl
*Main> main
42
Hello, world, from a library!
DefaultCompression
DefaultCompression
No connection string given, no PostgreSQL testing performed
*Main>

• Passing a connection string to test PostgreSQL functionality (and thus more complex C library interactions) in a REPL

$ PG_CONNECTION_STRING="postgresql://user:pass@localhost" bazel run //example-service:impl@repl
INFO: Analyzed target //example-service:impl@repl (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //example-service:impl@repl up-to-date:
  bazel-bin/example-service/impl@repl@repl
INFO: Elapsed time: 0.122s, Critical Path: 0.01s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
GHCi, version 8.6.5: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( example-service/impl/Main.hs, interpreted )
Ok, one module loaded.
Ok, one module loaded.
Loaded GHCi configuration from .../bazel-out/k8-fastbuild/bin/example-service/ghci-repl-script-impl@repl
*Main> main
42
Hello, world, from a library!
DefaultCompression
DefaultCompression
Testing PostgreSQL
2
*Main>

• Loading a REPL for a Haskell binary and a library it depends on

$ bazel run //example-service:impl-ifc-repl
INFO: Analyzed target //example-service:impl-ifc-repl (0 packages loaded, 1 target configured).
INFO: Found 1 target...
Target //example-service:impl-ifc-repl up-to-date:
  bazel-bin/example-service/impl-ifc-repl@repl
INFO: Elapsed time: 0.148s, Critical Path: 0.02s
INFO: 0 processes.
INFO: Build completed successfully, 2 total actions
INFO: Build completed successfully, 2 total actions
GHCi, version 8.6.5: http://www.haskell.org/ghc/  :? for help
[1 of 2] Compiling Example          ( example-service/ifc/Example.hs, interpreted )
[2 of 2] Compiling Main             ( example-service/impl/Main.hs, interpreted )
Ok, two modules loaded.
Ok, two modules loaded.
Loaded GHCi configuration from .../bazel-out/k8-fastbuild/bin/example-service/ghci-repl-script-impl-ifc-repl
*Example *Main>

• Building Haddock documentation for a Haskell library

$ bazel build //example-service:impl-ifc-docs
INFO: Analyzed target //example-service:impl-ifc-docs (48 packages loaded, 3026 targets configured).
INFO: Found 1 target...
INFO: From HaskellHaddock //example-service:ifc:
Loaded package environment from bazel-out/k8-fastbuild/bin/example-service/compile-package_env-ifc
Loaded package environment from bazel-out/k8-fastbuild/bin/example-service/compile-package_env-ifc
Loaded package environment from bazel-out/k8-fastbuild/bin/example-service/compile-package_env-ifc
Target //example-service:impl-ifc-docs up-to-date:
  bazel-bin/example-service/impl-ifc-docs/base-4.12.0.0
  bazel-bin/example-service/impl-ifc-docs/ghc-prim-0.5.3
  bazel-bin/example-service/impl-ifc-docs/integer-gmp-1.0.2.0
  bazel-bin/example-service/impl-ifc-docs/mtl-2.2.2
  bazel-bin/example-service/impl-ifc-docs/transformers-0.5.6.2
  bazel-bin/example-service/impl-ifc-docs/example-serviceZSifc
  bazel-bin/example-service/impl-ifc-docs/index
INFO: Elapsed time: 1.854s, Critical Path: 0.57s
INFO: 2 processes: 2 linux-sandbox.
INFO: Build completed successfully, 3 total actions

• Booting up GHCIDE directly for local development (check out hie.yaml and .hie-bios)

$ ghcide

• Booting up an editor or IDE that will start GHCIDE automatically (e.g. VSCode)

$ code .