Tools for specifying and parsing configurations

Version on this page:0.2.15
LTS Haskell 22.29:0.7.0
Stackage Nightly 2024-07-13:0.7.0
Latest on Hackage:0.7.0

See all snapshots configuration-tools appears in

MIT licensed and maintained by Lars Kuhtz
This version can be pinned in stack with:configuration-tools-0.2.15@sha256:a93930ca685103e43b69e069a4b7c3d38c4c8b48261716926327720b42feb9d8,5698

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

The main features are

  1. compositional configuration management with integration of command line option parsing and configuration files, validation of configurations, and loading of configurations from remote locations,
  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, and
  3. a set of types for configuration of HTTP services and clients along with aeson instances and command line option parsers.

The ultimate goal for this package is a general framework for compositional configuration management for software components. Instead of designing such a framework from scratch the approach of this package is to first explore design and implementation patterns based on practical examples and by gluing together existing technology.

Therefor at the current state this package mostly provides operators and coding patterns for writing stylish boilerplate code.

Once we feel that the developed patterns cover a sufficient portion of real world requirements we plan to rewrite this package such that the boilerplate is hidden behind a clean and simple DSL.


Assuming that you have a recent version version of GHC and Cabal installed in your system this package can be install from Hackage via

cabal install configuration-tools

If you don’t need support for remote configuration files this package can be build with a much smaller set of dependencies via

cabal install -f-remote-configs configuration-tools

The package can be tested via

git clone
cd hs-configurationt-tools
cabal configure --enable-tests
cabal build
cabal test

If you have issues building this package, first ensure that the installed version of the Cabal library matches the version that is used by your cabal binary. You may compare the results of cabal info Cabal and cabal --version.

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.

Optionally, a function for validating the configuration value may be provided.

The package provides operators and functions that make the implementation 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 –possibly partial– configuration in YAML or JSON format. The file location can be provided either as a local file system path or as a remote HTTP or HTTPS URL. In addition a list of static configuration file locations can be defined in the code.

    If this option is provided more than a single time the configuration files are loaded in the order as the respective options appear on the command line, where settings that are loaded later have precedence over earlier settings. Files from static locations are loaded before files that are specified on the command line.

  • 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.

As long as the package wasn’t build with -f-remote-configs the following two options are available. They affect how configuration files are loaded from remote URLs.

  • --config-https-insecure=true|false Bypass certificate validation for all HTTPS connections to all services.

  • --config-https-allow-cert=HOSTNAME:PORT:FINGERPRINT Unconditionally trust the certificate for connecting to the service.

The operators provided in this package 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 explicitly. 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 type synonyms for lenses. If you import Control.Lens you should hide Lens and 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 parenthesis in applicative style code.

instance FromJSON (Auth -> Auth) where
    parseJSON = withObject "Auth" $ \o -> 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 = 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 definitions 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 -> 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 = id
    <$< auth %:: pAuth
    <*< domain .:: strOption
        % long "domain"
        <> short 'd'
        <> help "HTTP domain"
    <*< path .:: strOption
        % long "path"
        <> 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
        $ "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 chosen 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 point-wise as a mapping from constructor parameters to values. An update for a sum type must take the constructor context into account. 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.

The module Configuration.Utils.Maybe provides tools for dealing with Maybe values.

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 package.

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 license of the application and exits.

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

trivial-0.2.6 (package configuration-tools-0.2.6 revision 2cc860c)
Copyright (c) 2014 AlephCloud, Inc.

Author: Lars Kuhtz <[email protected]>
License: MIT
Build with: ghc-7.8.3 (x86_64-osx)
Build flags:
Optimisation: normal

    Cabal- [BSD3, 2003-2006, Isaac Jones 2005-2011, Duncan Coutts]
    MonadRandom-0.3 [OtherLicense]
    aeson- [BSD3, (c) 2011-2014 Bryan O'Sullivan (c) 2011 MailRank, Inc.]
    ansi-terminal- [BSD3]
    ansi-wl-pprint- [BSD3]
    array- [BSD3]
    attoparsec- [BSD3]
    base- [BSD3]
    base-unicode-symbols- [BSD3, 2009–2011 Roel van Dijk <[email protected]>]
    bifunctors- [BSD3, Copyright (C) 2008-2013 Edward A. Kmett]
    rts-1.0 [BSD3]
    bytestring- [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.]
    case-insensitive- [BSD3, 2011 Bas van Dijk]
    comonad-4.2.2 [BSD3, Copyright (C) 2008-2014 Edward A. Kmett, Copyright (C) 2004-2008 Dave Menendez]
    conduit- [MIT]
    containers- [BSD3]
    contravariant-1.2 [BSD3, Copyright (C) 2007-2014 Edward A. Kmett]
    deepseq- [BSD3]
    directory- [BSD3]
    distributive-0.4.4 [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    dlist-0.7.1 [BSD3, 2006-2009 Don Stewart, 2013 Sean Leather]
    either-4.3.1 [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- [BSD3]
    free-4.9 [BSD3, Copyright (C) 2008-2013 Edward A. Kmett]
    ghc-prim- [BSD3]
    hashable- [BSD3]
    integer-gmp- [BSD3]
    lifted-base- [BSD3, (c) 2011-2012 Bas van Dijk, Anders Kaseorg]
    mmorph-1.0.4 [BSD3, 2013 Gabriel Gonzalez]
    monad-control- [BSD3, (c) 2011 Bas van Dijk, Anders Kaseorg]
    mtl-2.2.1 [BSD3]
    nats-0.2 [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    old-locale- [BSD3]
    optparse-applicative- [BSD3, (c) 2012-2014 Paolo Capriotti <[email protected]>]
    prelude-extras-0.4 [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    pretty- [BSD3]
    primitive- [BSD3, (c) Roman Leshchinskiy 2009-2012]
    process- [BSD3]
    profunctors- [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    random- [BSD3]
    resourcet- [BSD3]
    safe-0.3.6 [BSD3, Neil Mitchell 2007-2014]
    scientific- [BSD3]
    semigroupoids-4.2 [BSD3, Copyright (C) 2011-2013 Edward A. Kmett]
    semigroups-0.15.3 [BSD3, Copyright (C) 2011-2014 Edward A. Kmett]
    stm-2.4.3 [BSD3]
    syb-0.4.2 [BSD3]
    tagged-0.7.2 [BSD3, 2009-2013 Edward A. Kmett]
    template-haskell- [BSD3]
    text- [BSD3, 2009-2011 Bryan O'Sullivan, 2008-2009 Tom Harper]
    time-1.4.2 [BSD3]
    transformers- [BSD3]
    transformers-base-0.4.3 [BSD3, 2011 Mikhail Vorozhtsov <[email protected]>, Bas van Dijk <[email protected]>]
    transformers-compat- [BSD3, Copyright (C) 2012 Edward A. Kmett]
    unix- [BSD3]
    unordered-containers- [BSD3, 2010-2014 Johan Tibell 2010 Edward Z. Yang]
    vector- [BSD3, (c) Roman Leshchinskiy 2008-2012]
    void-0.6.1 [BSD3, Copyright (C) 2008-2013 Edward A. Kmett]
    yaml- [BSD3]

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.


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

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

  • Come up with a story 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.

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

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

  • Nicer error messages if parsing fails.

  • Support JSON encoded configuration files.

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

  • Include default values in help message.

  • Use ‘helpDoc’ to highlight “meta-options”, like options that enable optional configuration values through usage of the maybeOption function.

  • Add functionality optparse-applicative that allows to group options.



  • Support for http-client >= 0.5


  • Support for GHC-8 and Cabal-1.24


  • Eliminate most compiler warnings when compiling with GHC-7.10.

  • Bump lower bound on the version of optparse-applicative to This avoids issues when building with transformers-compat.

  • Reduce compilation time with text< by avoiding usage of toCaseFold from the case-insensitive package.

  • Dropped dependency on the error package.

  • Issue 43: Support detection of the version control system when the package directory and thus the cabal file is in sub-directory of the repository.


  • Added support for transformers- This changes allows usage of configuration-tools along with packages that depend on ghc, which in turn depends on transformers-


  • Added support for Cabal-1.18. This is supposed to make the build more robust and simplify integration with existing build infrastructure and other packages.


  • Moved all internal APIs to the Internal name space. Exposing them in 0.2.9 was considered a bug.

  • Configuration files can be formatted either as JSON or as YAML. For remote configuration files the HTTP Content-Type header is used to determine the format, for local files the file suffix is used. The default format is YAML.

  • Set the HTTP accept header for JSON and YAML when requesting remote configuration files.


  • Use tight constraint for all validation functions. Previously the constraint where unnecessarily restrictive.

  • Added updateProperty function that generalized %.: in the same way as setProperty generalizes ..:.

  • Added a validation function for configuration file arguments.

  • Allow usage of more than a single --config-file option on the command line.

  • Support for static configuration file locations. Configuration files can be marked as required or optional.

  • Support for loading of configuration files form HTTP and HTTPS URLs. There are new flags for disabling validation of SSL certificates and white listing SSL certificates based on their fingerprint.

  • Added tools for updating configurations with a monoid instance.

  • Added two new option parsers for boolean flags.

    • The boolOption_ parser uses the syntax --feature and --no-feature to enable and respectively disable a feature.

    • The enableDisableFlag parser uses the syntax --enable-feature and --disable-feature to enable and respectively disable a feature.

  • Refactored the module layout. The API of the existing modules is is backward compatible, but a lot of code got moved into submodules.

  • Improved documentation.

  • Improved test suite.


  • Added validation functions for Boolean values, numeric values and orders.

  • Added operator !..: for parsing of configuration values that are required to be present in a configuration file, thus preventing the default value from being used.

  • More consistent usage of case in metavar values.

  • Drop support for optparse-applicative < 0.10.


  • Added view function for lenses to Configuration.Utils.Internal.

  • Added support for validation of configuration values.

  • Added module Configuration.Utils.Validation that provides primitives for validating different basic configuration values.


  • For git repositories include also light-weight (non-annotated) tags in the version description.

  • Added new function boolReader, boolOption, fileOption, and eitherReadP to Utils.

  • Added new function maybeOption and improved documentation about Maybe config values.

  • Included optimisation level into long info.


  • Configuration.Utils.Setup: export mkPkgInfoModules function that modifies a given UserHooks record to generate an PkgInfo module during configuration.

  • Support for optparse-applicative >= 0.10.


  • Configuration.Utils.Setup: fixed generation of PkgInfo module for package configurations with explicit flags.

  • Improved documentation for Maybe values.


  • Show the help options in the options summary message.

  • Add -? as short version of for --help in addition to -h.

  • Remove showHelpOnError and disambiguate from option parser preferences.

  • Added file with installation instructions for windows to the package.


  • Add Lens piOptionParserAndDefaultConfiguration that gives simultaneous accesses to piOptionParser and piDefaultConfiguration which allows changing the type parameter of ProgramInfo a.

  • Introduce function setProperty. It is used as the ..: operator but allows to specify a custom parser for the property value instead of the default parseJSON from the FromJSON instance.

  • Introduce operators (<*<), (>*>), (<$<), (>$>) and deprecate (⊙) and (<.>).


  • Fix build with GHC-7.6 by relaxing lower bounds on some dependencies.


First release.