A module for producing forward-defined build systems, in contrast to standard backwards-defined build systems such as shake. Based around ideas from fabricate. As an example:

import Development.Shake
import Development.Shake.Forward
import Development.Shake.FilePath

main = shakeArgsForward shakeOptions $ do
    contents <- readFileLines "result.txt"
    cache $ cmd "tar -cf result.tar" contents

Compared to backward-defined build systems (such as normal Shake), forward-defined build systems tend to be simpler for simple systems (less boilerplate, more direct style), but more complex for larger build systems (requires explicit parallelism, explicit sharing of build products, no automatic command line targets). As a general approach for writing forward-defined systems:

• Figure out the sequence of system commands that will build your project.
• Write a simple Action that builds your project.
• Insert cache in front of most system commands.
• Replace most loops with forP, where they can be executed in parallel.
• Where Haskell performs real computation, if zero-build performance is insufficient, use cacheAction.

All forward-defined systems use AutoDeps, which requires fsatrace to be on the $PATH. You can obtain fsatrace from https://github.com/jacereda/fsatrace. You must set shakeLintInside to specify where AutoDeps will look for dependencies - if you want all dependencies everywhere use [""].

This module is considered experimental - it has not been battle tested. There are now a few possible alternatives in this space:

• Pier http://hackage.haskell.org/package/pier/docs/Pier-Core-Artifact.html (built on Shake).
• Rattle https://github.com/ndmitchell/rattle (by the same author as Shake).
• Stroll https://github.com/snowleopard/stroll.