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A Dynamic Reconfiguration Library for Haskell Programs

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README.mkd

Dyre - A Dynamic Reconfiguration Library for Haskell

Dyre implements a basic interface for dynamically recompiling Haskell programs with new configurations. The inspiration for all this is, of course, Xmonad's reconfiguration functionality. It is similar in usage to the HConf library which was written for the Yi editor.

Basic Usage

The main interface to the Dyre library consists of three items: a datatype for configuration data, a set of default values, and a function which makes that data into an entry-point function.

A complete, working example can be seen here:

-- DyreExample.hs --
module DyreExample ( dyreExample ) where

import qualified Config.Dyre as Dyre

confError cfgMessage error = "Error:" ++ error ++ "\n" ++ cfgMessage

realMain message = do
    putStrLn "Entered Main Function"
    putStrLn message

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

This code defines a simple library which will display a message on stdout. It uses Dyre to provide configuration of what exactly the message is. The default configuration could look something like this:

-- Main.hs --
import DyreExample
main = dyreExample "This is the default configuration"

While a user's custom configuration would be more like this:

-- dyreExample.hs --
import DyreExample
main = dyreExample "This is a custom configuration"

This example can be run without installing. Simply run Main.hs[^1] with the argument '--dyre-debug', to tell it to search for a configuration file in the current directory. If the custom configuration exists, it will be compiled and executed, or the default message will be displayed if there is no custom configuration.

The Dyre.wrapMain function is how Dyre is meant to be invoked. It is passed a set of parameters which configure how it operates. Most parameters have some good defaults defined in Dyre.defaultParams, except for the three defined in the above code. For a program to successfully run, those three parameters must be overridden.

The Dyre.projectName element is used to search for a custom configuration, the Dyre.showError element is called with compile errors to store the information in the program configuration, and Dyre.realMain is the function which is ultimately the main program entry point.

For the other parameters Dyre uses, consult the Config.Dyre.Params module documentation.

[^1] You can compile it with ghc --make, or simply use runhaskell Main.hs.

Restarting and Persistent State

Restarting is handled by the Config.Dyre.Relaunch module. To simply restart, discarding all state information, use the relaunchMaster function.

If the state needs to be preserved, there are two pairs of functions available. When your program starts, you will want to use ether the restoreTextState or the restoreBinaryState function. Both of these take a default state which is returned when there is no persisted state to use.

Then to restart and persist the state, use the function relaunchWithTextState or relaunchWithBinaryState corresponding to your chosen restore function. The state will be persisted and your program restarted for you.

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