Speeding up your cabal builds - Part II

Last time, I blogged about how linking your binaries against an internal library might speed up your cabal builds. This time, I show how you can avoid building certain binaries at all.

In our company, we work at a rather large Haskell project. The cabal file specifies more then ten binaries, so it takes rather long to build all of them. But often, you only need one or two of these binaries, so building them all is a waste of time.

Unfortunately, cabal does not allow you to build only a subset of your binaries. One workaround is the set the buildable flag in your .cabal file to false for the binaries you don’t want to build. However, this approach is rather unflexible because you need to edit the .cabal file and do a cabal configure after every change.

The solution I present in this article allows you to specify the binaries to build as arguments to the cabal build command. For example, if you want to build only binary B, you invoke cabal as cabal build B and cabal only builds binary B.

To get this working, all you need to do is writing a custom Setup.hs file:

import Data.List
import System.Exit
import Control.Exception

import Distribution.Simple
import Distribution.Simple.Setup
import Distribution.PackageDescription hiding (Flag)
import Distribution.PackageDescription.Parse
import Distribution.Verbosity (normal)

_CABAL_FILE_ = "DociGateway.cabal"

-- enable only certain binaries (specified on the commandline)                                                                                   
myPreBuildHook ::  Args -> BuildFlags -> IO HookedBuildInfo
myPreBuildHook [] flags = return emptyHookedBuildInfo
myPreBuildHook args flags =
    do let verbosity = case buildVerbosity flags of
                         Flag v -> v
                         NoFlag -> normal
       descr <- readPackageDescription verbosity _CABAL_FILE_
       let execs = map fst (condExecutables descr)
           unbuildableExecs = execs \\ args
       mapM_ (checkExistingExec execs) args
       putStrLn ("Building only " ++ intercalate ", " args)
       return (Nothing, map (\e -> (e, unbuildable)) unbuildableExecs)
    where
      unbuildable = emptyBuildInfo { buildable = False }
      checkExistingExec all x =
          if not (x `elem` all)
             then do putStrLn ("Unknown executable: " ++ x)
                     throw (ExitFailure 1)
             else return ()

main = defaultMainWithHooks $ simpleUserHooks { preBuild = myPreBuildHook }

That’s all! Don’t forget the set the Build-Type in your .cabal file to Custom. I’ve tested this approach with cabal-install version 0.8.2, using version 1.8.6 of the Cabal library.

Happy hacking and have fun!

Author: Stefan Wehr

Speeding up your cabal builds

Every waited too long for your cabal builds to finish? If that’s because you have multiple executable sections in your .cabal file, then there might be a solution.

By default, cabal rebuilds all relevant object files for each executable in separation. In other words, object files are not shared between executables. So if you have n executables and m source files, then cabal needs n * m compilation steps plus n link steps to rebuild the executables, no matter whethe any source file contributes to multiple executables.

Starting with cabal 1.8, there is a better solution, provided your executables have some source files in common. In this case, you might build a library from these common source files and then link the executables against the library. In the example above, if all n executables use the same set of m source files, then you end up with m compilation steps plus n + 1 link steps. Sounds good, doesn’t it?!

Here is a simple .cabal file that demonstrates how linking against an internal library works:

Name:                test
Version:             0.1
Synopsis:            test package for linking against internal libraries
Author:              Stefan Wehr
Build-type:          Simple
Cabal-version:       >=1.8 -- IMPORTANT

Library
  Hs-source-dirs: lib -- IMPORTANT
  Exposed-modules: A
  Build-Depends: base >= 4

Executable test-exe
  Build-depends: base >= 4, test, -- link against the internal library
  Main-is: Main.hs -- imports A
  Hs-source-dirs: prog  -- IMPORTANT

There are some things to consider:

• The Cabal-Version must be greater or equal 1.8.
• The library and the executable must not use common source directories, otherwise the compiler does not pick the library but recompiles the source files.
• The library must be mentioned in the Build-depends of the executable

Running cabal build now gives the following output:

Building test-0.1...
[1 of 1] Compiling A                ( lib/A.hs, dist/build/A.o )
Registering test-0.1...
[1 of 1] Compiling Main             ( prog/Main.hs, dist/build/test-exe/test-exe-tmp/Main.o )
Linking dist/build/test-exe/test-exe ...

No rebuilding of A when compiling Main!!!

This feature of cabal isn’t mentioned in the manual, at least I didn’t find it. Further, there seems to be no changelog for cabal. I found out about this feature by browsing the bug tracker for cabal. Is there a better way to get informed of new features of cabal?

Note: I successfully tested this with cabal-install version 0.8.2 (cabal library 1.8.0.4). I couldn’t get it to work with cabal-install version 0.8.0.

Author: Stefan Wehr