Configuration

Overview

In the build process section, we mentioned the elaborated install plan which is a detailed plan on how exactly to build all the packages in your build. There are cases in which we want to modify the configuration for certain packages in our build.

cabal.project Overview

While the .cabal specifies how to build a single package, a cabal.project specifies how to build all the packages in your project. In most simple use cases, you never have to worry about this file since it has smart defaults. The default is to automatically pick up packages - subfolders in your project with .cabal files.

Optimized Packages

When you're building a release version of your app, you may want to build with optimizations turned on for all your dependencies.

cabal.project

packages: .

all-packages
  optimization: 2

The all-packages section apply configuration to all packages in your build, both local and external.

Customizing a Single Package

Say you want to enable or disable a flag for a particular package in your build. You can use the package section.

packages: .

package aeson
  flag: +old-locale

Freezing Your Dependencies

To ensure full reproducibility of the build for your app, it is a good idea to freeze the dependencies so that your project build occurs in exactly the same way every time, independent of Hackage/Etlas Index changes.

$ etlas freeze

This will create a file called cabal.project.freeze in the current folder that contains a list of hard constraints for all the packages in your build. A freeze file ensures that the outcome dependency resolution is stable.

In practice, you should not freeze dependencies for libraries.

Next Module

In the next module, we will present a full reference of .cabal files and cabal.project files.