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copied!<p>Let's assume you have a <code>mylib</code> library, and <code>mylib-commandline</code> and <code>mylib-server</code> executables.</p> <p>You use <code>hs-source-dirs</code> for the library and each executable so that each has their own project root, avoiding double compilation:</p> <pre><code>mylib/ # Project root mylib.cabal src/ # Root for the library tests/ mylib-commandline/ # Root for the command line utility + helper modules mylib-server/ # Root for the web service + helper modules </code></pre> <p>Full directory layout:</p> <pre><code>mylib/ # Project root mylib.cabal src/ # Root for the library Web/ Mylib.hs # Main library module Mylib/ ModuleA # Mylib.ModuleA ModuleB # Mylib.ModuleB tests/ ... mylib-commandline/ # Root for the command line utility Main.hs # "module Main where" stub with "main = Web.Mylib.Commandline.Main.main" Web/ Mylib/ Commandline/ Main.hs # CLI entry point Arguments.hs # Programm command line arguments parser mylib-server/ # Root for the web service Server.hs # "module Main where" stub with "main = Web.Mylib.Server.Main.main" Web/ Mylib/ Server/ Main.hs # Server entry point Arguments.hs # Server command line arguments parser </code></pre> <p>The <em>stub-like</em> entry point file <code>mylib-commandline/Main.hs</code> looks like this:</p> <pre><code>module Main where import qualified Web.Mylib.Server.Main as MylibServer main :: IO () main = MylibServer.main </code></pre> <p>You need them because an <code>executable</code> must start on a module simply called <code>Main</code>.</p> <p>Your <code>mylib.cabal</code> looks like this:</p> <pre><code>library hs-source-dirs: src exposed-modules: Web.Mylib Web.Mylib.ModuleA Web.Mylib.ModuleB build-depends: base >= 4 && <= 5 , [other dependencies of the library] executable mylib-commandline hs-source-dirs: mylib-commandline main-is: Main.hs other-modules: Web.Mylib.Commandline.Main Web.Mylib.Commandline.Arguments build-depends: base >= 4 && <= 5 , mylib , [other depencencies for the CLI] executable mylib-server hs-source-dirs: mylib-server main-is: Server.hs other-modules: Web.Mylib.Server.Main build-depends: base >= 4 && <= 5 , mylib , warp >= X.X , [other dependencies for the server] </code></pre> <p><code>cabal build</code> will build the library and the two executables without double compilation of the library, because each is in their own <code>hs-source-dirs</code> and the executables depend on the library.</p> <p>You can still run the executables with <code>runghc</code> from your project root, using the <code>-i</code> switch to tell where it shall look for modules (using <code>:</code> as separator):</p> <pre><code>runhaskell -isrc:mylib-commandline mylib-commandline/Main.hs runhaskell -isrc:mylib-server mylib-server/Server.hs </code></pre> <p>This way, you can have a clean layout, executables with helper modules, and everything still works with <code>runhaskell</code>/<code>runghc</code> and <code>ghci</code>. To avoid typing this flag repeatedly, you can add something similar to</p> <pre><code>:set -isrc:mylib-commandline:mylib-server </code></pre> <p>to your <code>.ghci</code> file.</p> <hr> <p>Note that sometimes should split your code into separate packages, e.g. <code>mylib</code>, <code>mylib-commandline</code> and <code>mylib-server</code>.</p>

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