Overview

This package provides a collection of utils on top of the packages optparse-applicative, aeson, and yaml for configuring libraries and applications in a composable way.

The main features are

1. configuration management through integration of command line option parsing and configuration files and
2. a Setup.hs file that generates a PkgInfo module for each component of a package that provide information about the package and the build.

Configuration Management

The goal of this package is to make management of configurations easy by providing an idiomatic style of defining and deploying configurations.

For each data type that is used as a configuration type the following must be provided:

1. a default value,

2. a FromJSON instance that yields a function that takes a value and updates that value with the parsed values,

3. a ToJSON instance, and

4. an options parser that yields a function that takes a value and updates that value with the values provided as command line options.

The package provides operators and functions that make the implmentation of these requisites easy for the common case that the configuration is encoded mainly through nested records.

In addition to the user defined command line options the following options are recognized by the application:

--config-file, -c : parses the given file as a (partial) configuration in YAML format.

print-config, -p : configures the application and prints the configuration in YAML format to standard out and exits. The printed configuration is exactly the configuration that otherwise would be used to run the application.

--help, -h : prints a help message and exits.

The operators assume that lenses are provided for field of the configuration record types.

An complete usage example can be found in the file example/Example.hs of the cabal package.

Usage Example

Remark: there are unicode equivalents for some operators available in Configuration.Utils that lead to better aligned and more readable code.

We start with language extensions and imports.

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleInstances #-}

module Main
( main
) where

import Configuration.Utils
import Data.Monoid

Next we define the types that are used for the configuration of our application. In this contrived example these types define a simplified version of HTTP URLs.

data Auth = Auth
    { _user :: !String
    , _pwd :: !String
    }

We have to define lenses for the configuration types. Here we do it explicitely. Alternatively one could have used TemplateHaskell along with makeLenses from the module Control.Lens from the lens package.

user :: Functor f => (String -> f String) -> Auth -> f Auth
user f s = (\u → s { _user = u }) <$> f (_user s)

pwd :: Functor f => (String -> f String) -> Auth -> f Auth
pwd f s = (\p -> s { _pwd = p }) <$> f (_pwd s)

(Note, that the module Configuration.Utils defines its own Lens' type synonym. If you import Control.Lens you should hide Lens' from either module.)

We must provide a default value. If there is no reasonable default the respective value could, for instance, be wrapped into Maybe. Here we use the monoid identity value of the type.

defaultAuth :: Auth
defaultAuth = Auth
    { _user = ""
    , _pwd = ""
    }

Now we define an aeson FromJSON instance that yields a function that updates a given Auth value with the values from the parsed JSON value. The <.> operator is functional composition lifted for applicative functors and % is a version of $ with a different precedence that helps to reduce the use of paranthesis in applicative style code.

instance FromJSON (Auth -> Auth) where
    parseJSON = withObject "Auth" $ \o -> pure id
        <.> user ..: "user" % o
        <.> pwd ..: "pwd" % o

The ToJSON instance is needed to print the configuration (as YAML document) when the user provides the --print-config command line option.

instance ToJSON Auth where
    toJSON a = object
        [ "user" .= _user a
        , "pwd" .= _pwd a
        ]

Finally we define a command line option parser using the machinery from the optparse-applicative package. Similar to the FromJSON instance the parser does not yield a value directly but instead yields a function that updates a given Auth value with the value from the command line.

pAuth :: MParser Auth
pAuth = pure id
    <.> user .:: strOption
        % long "user"
        <> help "user name"
    <.> pwd .:: strOption
        % long "pwd"
        <> help "password for user"

You may consult the documentation of the optparse-applicative package for further information on how to define command line options.

The following definitons for the HttpURL are similar to definitions for the Auth type above. In addition it is demonstrated how to deal with nested configuration types. Mainly the usage of ..: is replaced by %.: and .:: is replaced by %::.

data HttpURL = HttpURL
    { _auth :: !Auth
    , _domain :: !String
    , _path :: !String
    }

auth :: Functor f => (Auth -> f Auth) -> HttpURL -> f HttpURL
auth f s = (\u → s { _auth = u }) <$> f (_auth s)

domain :: Functor f => (String -> f String) -> HttpURL -> f HttpURL
domain f s = (\u → s { _domain = u }) <$> f (_domain s)

path :: Functor f => (String -> f String) -> HttpURL -> f HttpURL
path f s = (\u → s { _path = u }) <$> f (_path s)

defaultHttpURL :: HttpURL
defaultHttpURL = HttpURL
    { _auth = defaultAuth
    , _domain = ""
    , _path = ""
    }

instance FromJSON (HttpURL -> HttpURL) where
    parseJSON = withObject "HttpURL" $ \o -> pure id
        <.> auth %.: "auth" % o
        <.> domain ..: "domain" % o
        <.> path ..: "path" % o

instance ToJSON HttpURL where
    toJSON a = object
        [ "auth" .= _auth a
        , "domain" .= _domain a
        , "path" .= _path a
        ]

pHttpURL :: MParser HttpURL
pHttpURL = pure id
    <.> auth %:: pAuth
    <.> domain .:: strOption
        % long "domain"
        <> short 'd'
        <> help "HTTP domain"
    <.> path .:: strOption
        % long "path"
        <> short 'p'
        <> help "HTTP URL path"

Now that everything is set up the configuration can be used to create a ProgramInfo value. The ProgramInfo value is than use with the runWithConfiguratin function to wrap a main function that takes an HttpURL argument with configuration file and command line parsing.

mainInfo :: ProgramInfo HttpURL
mainInfo = programInfo "HTTP URL" pHttpURL defaultHttpURL

main :: IO ()
main = runWithConfiguration mainInfo $ \conf -> do
    putStrLn
        $ "http://"
        <> (_user . _auth) conf
        <> ":"
        <> (_pwd . _auth) conf
        <> "@"
        <> _domain conf
        <> "/"
        <> _path conf

Using Sum Types as Configuration Types

Sum types can not be used as configuration types in the same way as product types. The reason is that the nondeterminism in the choice of a term for the type is not restricted to the choosen constructor arguments but in addition there is non-determinism in the choice of the constructor, too.

An update function for a product type can be defined pointwise as a mapping from constructor parameters to values. An update for a sum type must take the constructor context into account. In terms of the lens library this is reflected by using Lenses for product types and Prisms for sum types. Therefore a configuration that defines an update function for a sum types must also specify the constructor context. Moreover, when applied to a given default value the function may not be applicable at all if the default value uses a different constructor context than what the update assumes.

For the future we plan to provide a general solution for configurations of sum types which would be based on the possibility to define default values for more than a single constructor. For now one must restrict configurations of sum types to yield constant values instead of point-wise (partial) updates. In practice this means that for a type a one has to provide an FromJSON instance for a and use the ..: operator. Similarly for the option parser one has to define a parser that yields an a and use it with the .:: operator.

Optional Configuration Values

For configuration values of type Maybe a, though being sum types, we provide an orphan¹ FromJSON instance of the form

instance (FromJSON a, FromJSON (a -> a)) => FromJSON (Maybe a -> Maybe a)

that has the following behavior:

If the parsed configuration value is 'Null' the resulting function constantly returns Nothing. Otherwise

• the function does an pointwise update using the FromJSON instance for a -> a when applied to Just a and
• the function uses the FromJSON instance for a to return the parsed a value when applied to 'Nothing'.

The FromJSON a instance may either require that the parsed configuration fully specifies the value of a (and raise a failure otherwise) or the FromJSON a instance may do an pointwise update of a hardcoded default value based on the existing FromJSON (a -> a) instance.

For instance, assuming that there is already an FromJSON instance for MyType -> MyType and a default value defaultMyType the following pattern can be used:

instance FromJSON MyType where
    parseJSON v = parseJSON v <*> defaultMyType

Package and Build Information

The module Configuration.Utils.Setup an example Setup.hs script that hooks into the cabal build process at the end of the configuration phase and generates a module with package information for each component of the cabal pacakge.

The modules are created in the autogen build directory where also the Path_ module is created by cabal's simple build setup. This is usually the directory ./dist/build/autogen.

For a library component the module is named just PkgInfo. For all other components the module is named PkgInfo_COMPONENT_NAME where COMPONENT_NAME is the name of the component with - characters replaced by _.

For instance, if a cabal package contains a library and an executable that is called my-app, the following modules are created: PkgInfo and PkgInfo_my_app.

Usage as Setup Script

There are two ways how this module can be used:

1. Copy the code of this module into a file called Setup.hs in the root directory of your package.

2. If the configuration-tools package is already installed in the system where the build is done, following code can be used as Setup.hs script:

   module Main (main) where
   
   import Configuration.Utils.Setup
   

With both methods the field Build-Type in the package description (cabal) file must be set to Custom:

Build-Type: Custom

Integration With Configuration.Utils

You can integrate the information provided by the PkgInfo modules with the command line interface of an application by importing the respective module for the component and using the runWithPkgInfoConfiguration function from the module Configuration.Utils as show in the following example:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleInstances #-}

module Main
( main
) where

import Configuration.Utils
import PkgInfo

instance FromJSON (() -> ()) where parseJSON _ = pure id

mainInfo :: ProgramInfo ()
mainInfo = programInfo "Hello World" (pure id) ()

main :: IO ()
main = runWithPkgInfoConfiguration mainInfo pkgInfo . const $ putStrLn "hello world"

With that the resulting application supports the following additional command line options:

--version, -v : prints the version of the application and exits.

--info, -i : prints a short info message for the application and exits.

--long-info : print a detailed info message for the application and exits. Beside component name, package name, version, revision, and copyright the message also contain information about the compiler that was used for the build, the build architecture, build flags, the author, the license type, and a list of all direct and indirect dependencies along with their licenses and copyrights.

--license : prints the text of the lincense of the application and exits.

Here is the example output of --long-info for the example examples/Trivial.hs from this package:

trivial-0.1 (package configuration-tools-0.1 revision 080c27a)
Copyright (c) 2014 AlephCloud, Inc.

Author: Lars Kuhtz <lars@alephcloud.com>
License: MIT
Homepage: https://github.com/alephcloud/hs-configuration-tools
Build with: ghc-7.8.2 (x86_64-osx)
Build flags:

Dependencies:
    Cabal-1.21.0.0 [BSD3, 2003-2006, Isaac Jones 2005-2011, Duncan Coutts]
    MonadRandom-0.1.13 [OtherLicense]
    aeson-0.7.0.6 [BSD3, (c) 2011-2014 Bryan O'Sullivan (c) 2011 MailRank, Inc.]
    ansi-terminal-0.6.1.1 [BSD3]
    ansi-wl-pprint-0.6.7.1 [BSD3]
    array-0.5.0.0 [BSD3]
    attoparsec-0.11.3.4 [BSD3]
    base-4.7.0.0 [BSD3]
    base-unicode-symbols-0.2.2.4 [BSD3, 2009–2011 Roel van Dijk <vandijk.roel@gmail.com>]
    rts-1.0 [BSD3]
    bytestring-0.10.4.0 [BSD3, Copyright (c) Don Stewart 2005-2009, (c) Duncan Coutts 2006-2013, (c) David Roundy 2003-2005, (c) Jasper Van der Jeugt 2010, (c) Simon Meier 2010-2013.]
    comonad-4.2 [BSD3, Copyright (C) 2008-2013 Edward A. Kmett, Copyright (C) 2004-2008 Dave Menendez]
    conduit-1.1.2.1 [MIT]
    containers-0.5.5.1 [BSD3]
    contravariant-0.5.1 [BSD3, Copyright (C) 2007-2013 Edward A. Kmett]
    deepseq-1.3.0.2 [BSD3]
    directory-1.2.1.0 [BSD3]
    distributive-0.4.3.2 [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    dlist-0.7.0.1 [BSD3, 2006-2009 Don Stewart, 2013 Sean Leather]
    either-4.1.2 [BSD3, Copyright (C) 2008-2014 Edward A. Kmett]
    errors-1.4.7 [BSD3, 2012, 2013 Gabriel Gonzalez]
    exceptions-0.6.1 [BSD3, Copyright (C) 2013-2014 Edward A. Kmett Copyright (C) 2012 Google Inc.]
    filepath-1.3.0.2 [BSD3]
    ghc-prim-0.3.1.0 [BSD3]
    hashable-1.2.2.0 [BSD3]
    integer-gmp-0.5.1.0 [BSD3]
    lifted-base-0.2.2.2 [BSD3, (c) 2011-2012 Bas van Dijk, Anders Kaseorg]
    mmorph-1.0.3 [BSD3, 2013 Gabriel Gonzalez]
    monad-control-0.3.3.0 [BSD3, (c) 2011 Bas van Dijk, Anders Kaseorg]
    mtl-2.1.3.1 [BSD3]
    nats-0.2 [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    old-locale-1.0.0.6 [BSD3]
    optparse-applicative-0.8.1 [BSD3, (c) 2012 Paolo Capriotti <p.capriotti@gmail.com>]
    pretty-1.1.1.1 [BSD3]
    primitive-0.5.3.0 [BSD3, (c) Roman Leshchinskiy 2009-2012]
    process-1.2.0.0 [BSD3]
    random-1.0.1.1 [BSD3]
    resourcet-1.1.2.2 [BSD3]
    safe-0.3.4 [BSD3, Neil Mitchell 2007-2014]
    scientific-0.3.2.0 [BSD3]
    semigroupoids-4.0.2 [BSD3, Copyright (C) 2011-2013 Edward A. Kmett]
    semigroups-0.14 [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    syb-0.4.1 [BSD3]
    tagged-0.7.2 [BSD3, 2009-2013 Edward A. Kmett]
    template-haskell-2.9.0.0 [BSD3]
    text-1.1.1.2 [BSD3, 2009-2011 Bryan O'Sullivan, 2008-2009 Tom Harper]
    time-1.4.2 [BSD3]
    transformers-0.3.0.0 [BSD3]
    transformers-base-0.4.2 [BSD3, 2011 Mikhail Vorozhtsov <mikhail.vorozhtsov@gmail.com>, Bas van Dijk <v.dijk.bas@gmail.com>]
    transformers-compat-0.1.1.1 [BSD3, Copyright (C) 2012 Edward A. Kmett]
    unix-2.7.0.1 [BSD3]
    unordered-containers-0.2.4.0 [BSD3, 2010-2014 Johan Tibell 2010 Edward Z. Yang]
    vector-0.10.9.1 [BSD3, (c) Roman Leshchinskiy 2008-2012]
    void-0.6.1 [BSD3, Copyright (C) 2008-2013 Edward A. Kmett]
    yaml-0.8.8.3 [BSD3]

Available options:
  -i,--info                Print program info message and exit
  --long-info              Print detailed program info message and exit
  -v,--version             Print version string and exit
  --license                Print license of the program and exit
  -h,--help                Show this help text
  -p,--print-config        Print the parsed configuration to standard out and
                           exit
  -c,--config-file FILE    Configuration file for backend services in YAML
                           fromat

Configuration Types for HTTP Services and Clients

The module Configuration.Utils.Http contains some types for configuring HTTP services and clients. Currently these types only provide the most basic configuration settings. This will probably be extended in the future. Feel free to submit patches for missing settings.

TODO

This package is in an early stage of development and more features are planned.

• Teach optparse-applicative to not print usage-message for info options.

• Simplify specification of Configuration data types by integrating the aeson instances and the option parser.

• Come up with a storry for sum types. We may use the following approach: The definition of the default should include alternate values for each constructor. Effectively, this means to map the sum type onto a product type by interpreting the summands as factors. For mapping back from the product type to the original sum type one has to provide a choice of the constructor. Intuitively, a sum type can be represented as a tree where the leafs partition the type into classes of value with the same constructors. By providing a default value for each such class partial configurations that are defined through point-wise updates can always be applied in a meaningful way.

  We may use GHC Generics to derive the type for representing default values for all constructure classes. We can then define an operator that allows to construct the generic default value by combining values for the different constructors of the original sum type.

  The definition of the JSON instances and option parsers would use prisms that would update a value only for supported constructor contexts. In addition we may provide a way to configure the choice of a particular constructor.

• Include help text as comments in YAML serialization of configuration values.

• Provide operators (or at least examples) for more scenarios (like required options)

• Nicer errors messages if parsing fails.

• Suport JSON encoded configuration files.

• Support mode where JSON/YAML parsing fails when unexpected properties are encountered.

• Loading of configurations from URLs.

• Include default values in help message.

1

Using an orphan instance is generally problematic but convenient in this case. It's unlike that such an instance is needed elsewhere. If this is an issue for you, please let me know. In that case we can define a new type for optional configuration values.