The endo package

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Package defines extra functions for Data.Monoid.Endo data type, and also generic endomorphism folding machinery. Generic endomorphism folding can be used for various purposes, including as a builder.

Here is an example how to use it with optparse-applicative package:

 data Verbosity = Silent | Normal | Verbose | Annoying
   deriving (Show)
 data Config = Config Verbosity FilePath
   deriving (Show)
 options :: Parser Config
 options = runIdentityT $ runEndo defaultConfig <$> options'
   where
     options' :: IdentityT Parser (Endo Config)
     options' = foldEndo
         <*> outputOption     -- IdentityT Parser (Maybe (E Config))
         <*> verbosityOption  -- IdentityT Parser (Maybe (E Config))
         <*> annoyingFlag     -- IdentityT Parser (E Config)
         <*> silentFlag       -- IdentityT Parser (E Config)
         <*> verboseFlag      -- IdentityT Parser (E Config)

     defaultConfig :: Config
     defaultConfig = Config Normal ""
 main :: IO ()
 main = execParser (info options fullDesc) >>= print
 ghci> :main -o an.out.put --annoying
 Config Annoying "an.out.put"

For details how individual option parsers look like see module Data.Monoid.Endo.Fold which contains other examples as well as this one.


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Properties

Versions0.1.0.0, 0.1.0.1, 0.1.0.2, 0.1.0.2, 0.2.0.0, 0.2.0.1, 0.3.0.0, 0.3.0.1
Change logChangeLog.md
Dependenciesbase (>=4.6 && <4.8), between (==0.9.*), transformers (>=0.3 && <0.5) [details]
LicenseBSD3
Copyright(c) 2013-2015 Peter Trško
AuthorPeter Trško
Maintainerpeter.trsko@gmail.com
CategoryData
Home pagehttps://github.com/trskop/endo
Bug trackerhttps://github.com/trskop/endo/issues
Source repositoryhead: git clone git://github.com/trskop/endo.git
this: git clone git://github.com/trskop/endo.git(tag 0.1.0.2)
UploadedMon Jan 19 15:54:19 UTC 2015 by PeterTrsko

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Flags

NameDescriptionDefaultType
pedanticPass additional warning flags to GHC.DisabledManual

Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info

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For package maintainers and hackage trustees

Readme for endo-0.1.0.2

Endo

Hackage Haskell Programming Language BSD3 License

Description

Endomorphism utilities.

Usage Examples

Lets define simple application Config data type as:

data Verbosity = Silent | Normal | Verbose | Annoying
  deriving (Show)

data Config = Config
    { _verbosity :: Verbosity
    , _outputFile :: FilePath
    }
  deriving (Show)

Now lets define setters for _verbosity and _outputFile:

setVerbosity :: Verbosity -> E Config
setVerbosity b cfg = cfg{_verbosity = b}

setOutputFile :: FilePath -> E Config
setOutputFile b cfg = cfg{_outputFile = b}

Note that E is defined in Data.Monoid.Endo module and it looks like:

type E a = a -> a

Its purpose is to simplify type signatures.

Now lets get to our first example:

example1 :: E Config
example1 = appEndo $ foldEndo
    &$ setVerbosity Annoying
    &$ setOutputFile "an.out.put"

Above example shows us that it is possible to modify Config as if it was a monoid, but without actually having to state it as such. In practice it is not always possible to define it as Monoid or at least a Semigroup. What usually works are endomorphisms, like in this example.

Now, FilePath has one pathological case, and that is "". There is a lot of ways to handle it. Here we will concentrate only few basic techniques to illustrate versatility of our approach.

-- | Trying to set output file to \"\" will result in keeping original value.
setOutputFile2 :: FilePath -> E Config
setOutputFile2 "" = id
setOutputFile2 fp = setOutputFile fp

example2 :: E Config
example2 = appEndo $ foldEndo
    &$ setVerbosity Annoying
    &$ setOutputFile2 "an.out.put"

Same as above, but exploits instance AnEndo a => AnEndo Maybe a:

setOutputFile3 :: FilePath -> Maybe (E Config)
setOutputFile3 "" = Nothing
setOutputFile3 fp = Just $ setOutputFile fp

example3 :: E Config
example3 = appEndo $ foldEndo
    &$ setVerbosity Annoying
    &$ setOutputFile3 "an.out.put"

Following example uses common pattern of using Either as error reporting monad. This approach can be easily modified for arbitrary error reporting monad.

setOutputFile4 :: FilePath -> Either String (E Config)
setOutputFile4 "" = Left "Output file: Empty file path."
setOutputFile4 fp = Right $ setOutputFile fp

example4 :: Either String (E Config)
example4 = appEndo <&$> foldEndo
    <*> pure (setVerbosity Annoying)
    <*> setOutputFile4 "an.out.put"

Notice, that above example uses applicative style. Normally when using this style, for setting record values, one needs to keep in sync order of constructor arguments and order of operations. Using foldEndo (and its dual dualFoldEndo) doesn't have this restriction.

Instead of setter functions one may want to use lenses (in terms of lens package):

verbosity :: Lens' Config Verbosity
verbosity = _verbosity ~@@^> \s b -> s{_verbosity = b}

outputFile :: Lens' Config FilePath
outputFile = _outputFile ~@@^> \s b -> s{_outputFile = b}

Now setting values of Config would look like:

example5 :: E Config
example5 = appEndo $ foldEndo
    &$ verbosity  .~ Annoying
    &$ outputFile .~ "an.out.put"

Probably one of the most interesting things that can be done with this module is following:

instance AnEndo Verbosity where
    type EndoOperatesOn Verbosity = Config
    anEndo = Endo . set verbosity

newtype OutputFile = OutputFile FilePath

instance AnEndo OutputFile where
    type EndoOperatesOn OutputFile = Config
    anEndo (OutputFile fp) = Endo $ outputFile .~ fp

example6 :: E Config
example6 = appEndo $ foldEndo
    &$ Annoying
    &$ OutputFile "an.out.put"

Using with optparse-applicative

This is a more complex example that defines parser for optparse-applicative built on top of some of the above definitions:

options :: Parser Config
options = runIdentityT $ runEndo defaultConfig <$> options'
  where
    options' :: IdentityT Parser (Endo Config)
    options' = foldEndo
        <*> outputOption     -- :: IdentityT Parser (Maybe (E Config))
        <*> verbosityOption  -- :: IdentityT Parser (Maybe (E Config))
        <*> annoyingFlag     -- :: IdentityT Parser (E Config)
        <*> silentFlag       -- :: IdentityT Parser (E Config)
        <*> verboseFlag      -- :: IdentityT Parser (E Config)

    defaultConfig :: Config
    defaultConfig = Config Normal ""

-- >>> :main -o an.out.put --annoying
-- Config {_verbosity = Annoying, _outputFile = "an.out.put"}
main :: IO ()
main = execParser (info options fullDesc) >>= print

Parsers for individual options and flags are wrapped in IdentityT, because there is no following instance:

instance FoldEndoArgs r => FoldEndoArgs (Parser r)

But there is:

instance (Applicative f, FoldEndoArgs r) => FoldEndoArgs (IdentityT f r)

Functions used by the above code example:

outputOption :: IdentityT Parser (Maybe (E Config))
outputOption =
    IdentityT . optional . option (set outputFile <$> parseFilePath)
    $ short 'o' <> long "output" <> metavar "FILE"
        <> help "Store output in to a FILE."
  where
    parseFilePath = eitherReader $ \s ->
        if null s
            then Left "Option argument can not be empty file path."
            else Right s

verbosityOption :: IdentityT Parser (Maybe (E Config))
verbosityOption =
    IdentityT . optional . option (set verbosity <$> parseVerbosity)
    $ long "verbosity" <> metavar "LEVEL" <> help "Set verbosity to LEVEL."
  where
    verbosityToStr = map toLower . Data.showConstr . Data.toConstr
    verbosityIntValues = [(show $ fromEnum v, v) | v <- [Silent .. Annoying]]
    verbosityStrValues =
        ("default", Normal) : [(verbosityToStr v, v) | v <- [Silent .. Annoying]]

    parseVerbosityError = unwords
        [ "Verbosity can be only number from interval"
        , show $ map fromEnum [minBound, maxBound :: Verbosity]
        , "or one of the following:"
        , concat . intersperse ", " $ map fst verbosityStrValues
        ]

    parseVerbosity = eitherReader $ \s ->
        case lookup s $ verbosityIntValues ++ verbosityStrValues of
            Just v  -> Right v
            Nothing -> Left parseVerbosityError

annoyingFlag :: IdentityT Parser (E Config)
annoyingFlag = IdentityT . flag id (verbosity .~ Annoying)
    $ long "annoying" <> help "Set verbosity to maximum."

silentFlag :: IdentityT Parser (E Config)
silentFlag = IdentityT . flag id (verbosity .~ Silent)
    $ short 's' <> long "silent" <> help "Set verbosity to minimum."

verboseFlag :: IdentityT Parser (E Config)
verboseFlag = IdentityT . flag id (verbosity .~ Verbose)
    $ short 'v' <> long "verbose" <> help "Be verbose."

Building Options

Contributions

Contributions, pull requests and bug reports are welcome! Please don't be afraid to contact author using GitHub or by e-mail.