dyre-0.6: Dynamic reconfiguration in Haskell



Dyre is a library for configuring your Haskell programs. Like Xmonad, programs configured with Dyre will look for a configuration file written in Haskell, which essentially defines a custom program configured exactly as the user wishes it to be. And since the configuration is written in Haskell, the user is free to do anything they might wish in the context of configuring the program.

Dyre places emphasis on elegance of operation and ease of integration with existing applications. The wrapMain function is the sole entry point for Dyre. When partially applied with a parameter structure, it wraps around the realMain value from that structure, yielding an almost identical function which has been augmented with dynamic recompilation functionality.

The Config.Dyre.Relaunch module provides the ability to restart the program (recompiling if applicable), and persist state across restarts, but it has no impact whatsoever on the rest of the library whether it is used or not.

A full example of using Dyre's recompilation and relaunching features is as follows:

-- DyreExample.hs --
module DyreExample where

import qualified Config.Dyre as Dyre

import Config.Dyre.Relaunch
import System.IO

data Config = Config { message :: String, errorMsg :: Maybe String }
data State  = State { bufferLines :: [String] } deriving (Read, Show)

defaultConfig :: Config
defaultConfig = Config "Dyre Example v0.1" Nothing

showError :: Config -> String -> Config
showError cfg msg = cfg { errorMsg = Just msg }

realMain Config{message = message, errorMsg = errorMsg } = do
    (State buffer) <- restoreTextState $ State []
    case errorMsg of
         Nothing -> return ()
         Just em -> putStrLn $ "Error: " ++ em
    putStrLn message
    mapM putStrLn . reverse $ buffer
    putStr "> " >> hFlush stdout
    input <- getLine
    case input of
         "exit" -> return ()
         "quit" -> return ()
         other  -> relaunchWithTextState (State $ other:buffer) Nothing

dyreExample = Dyre.wrapMain $ Dyre.defaultParams
    { Dyre.projectName = "dyreExample"
    , Dyre.realMain    = realMain
    , Dyre.showError   = showError

Notice that all of the program logic is contained in the DyreExample module. The main module of the program is absolutely trivial.

-- Main.hs --
module Main where
import DyreExample
main = dyreExample defaultConfig

When reading the above program, bear in mind exactly how much of the code is simply *program logic*. Dyre is designed to intelligently handle recompilation with a bare minimum of program modification.

Some mention should be made of Dyre's defaults. The defaultParams structure used in the example defines reasonable default values for several configuration items. In the absence of any other definitions, Dyre will default to outputting status messages to stderr, not hiding any packages during compilation, and passing no special options to GHC.

Also, Dyre will expect configuration files to be placed at the path '$XDG_CONFIG_HOME/<app>/<app>.hs', and it will store cache files in the '$XDG_CACHE_HOME/<app>/' directory. The System.Environment.XDG module will be used to determine these paths, so refer to it for behaviour on Windows platforms.



wrapMain :: Params cfgType -> cfgType -> IO ()Source

wrapMain is how Dyre recieves control of the program. It is expected that it will be partially applied with its parameters to yield a main entry point, which will then be called by the main function, as well as by any custom configurations.

data Params cfgType Source

This structure is how all kinds of useful data is fed into Dyre. Of course, only the projectName, realMain, and showError fields are really necessary. By using the set of default values provided as Config.Dyre.defaultParams, you can get all the benefits of using Dyre to configure your program in only five or six lines of code.




projectName :: String

The name of the project. This needs to also be the name of the executable, and the name of the configuration file.

configDir :: Maybe (IO FilePath)

The directory to look for a configuration file in.

cacheDir :: Maybe (IO FilePath)

The directory to store build files in, including the final generated executable.

realMain :: cfgType -> IO ()

The main function of the program. When Dyre has completed all of its recompilation, it passes the configuration data to this function and gets out of the way.

showError :: cfgType -> String -> cfgType

This function is used to display error messages that occur during recompilation, by allowing the program to modify its initial configuration.

hidePackages :: [String]

Packages that need to be hidden during compilation

ghcOpts :: [String]

Miscellaneous GHC compilation settings go here

statusOut :: String -> IO ()

A status output function. Will be called with messages when Dyre recompiles or launches anything. A good value is 'hPutStrLn stderr', assuming there is no pressing reason to not put messages on stderr.

defaultParams :: Params cfgTypeSource

A set of reasonable defaults for configuring Dyre. If the minimal set of fields are modified, the program will use the XDG-defined locations for configuration and cache files (see System.Environment.XDG.BaseDir for details), pass no special options to GHC, and will output status messages to stderr.

The fields that will have to be filled are projectName, realMain, and showError