Advanced Features

Dive into features for specialized use cases

Advanced Features

Conditional Configuration

Contents

Overview

As Eta evolves and your packages make use of newer features, you will the need to start worrying about backwards compatibility for your package.


Or maybe you just want to provide different flavors of your package that is built against different dependencies according to certain conditions.


To handle these and other user cases, Etlas provides conditional configuration which allows you to configure options when certain conditions hold.

if/else blocks

In your .cabal file, you can use if and else keywords to specify a condition under which the fields in the block should be included.


Example 1

if os(windows)
  cpp-options: -DWINDOWS

Example 2

if os(windows)
  build-depends: my-windows-library
else
  build-depends: unix-library

Condition Values

After simplification, a condition evaluates to true or false, where true means that condition holds and false means the condition doesn't hold.


Example 1

if true
  build-depends: base

is the same as


build-depends: base

Predicates

The following built-in predicates, or conditions, are available:


Predicate Argument Type Description (Returns true if...)
os os Current operating system matches the argument
arch arch Current architecture matches the argument
impl prog-constraint Program constraint in the argument holds with the build configuration
flag flag Flag has been assigned via CLI or cabal.project configuration

Each of the argument types is explained in detail below.


os

The operating system can be specified by the following values:


  • linux
  • osx
  • windows
  • solaris
  • freebsd
  • openbsd
  • netbsd

Example


if os(freebsd)
  cpp-options: -DPLATFORM_FREEBSD

arch

The architecture can be specified by the following values:


  • i386
  • x86_64
  • ppc
  • ppc64
  • sparc
  • alpha

Example


if arch(x86_64)
  cpp-options: -DENABLE_64BIT_OPTIMIZATIONS

prog-constraint

A program constraint takes the form [compiler-name] [version-range] where [compiler-name] can be the name of the compiler you want to check against and [version-range] has been specified in the previous section. Note that [version-range] is optional and if not present, just checks whether [compiler-name] is configured for the build.


Example 1


if impl(eta > 0.7.2.1)
  java-sources: java/eta-0.7.3/Utils.java
else
  java-sources: java/eta-0.7.2/Utils.java

Example 2


if impl(eta)
  cpp-options: -DCOMPILING_ETA

flag

This predicate is used to check whether a package flag has been evaluated to true or not.


Example


if flag(developer)
  cpp-options: -DDEVELOPMENT

Operators

The logical operators supported (in precedence order) is as follows:


  • !: Logical NOT
  • &&: Logical AND
  • ||: Logical OR

Example

if impl(eta) && !os(windows)
  eta-options: -funix-optimizations

Next Section

In the next section, we will cover all ways you can configure a cabal.project file.

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