Composable Command Line Parsing with optparse-applicative

There are many solutions for parsing command line arguments in Haskell. Personally I like optparse-applicative, because, like the title suggests, you can compose parsers out of smaller pieces.

I have written command line parsers for postgresql-simple's database connection info many times. Faced with the prospect of doing it again I opted to make this library, which is also a single literate Haskell file. This way I could reuse it in web servers, db migrators, db job runners ... those are all the examples I could think of ... just trust me, it's worth it.

Outline

• The "Partial" Option Types
• The Composable Parser
• The Complete Option
• Option "completion"
• The Option Parser
• The Runner
• The Tests

Standard Intro Statements to Ignore

{-| A resuable optparse-applicative parser for creating a postgresql-simple
   'Connection'
-}
{-# LANGUAGE RecordWildCards, LambdaCase, DeriveGeneric, DeriveDataTypeable #-}
{-# LANGUAGE GeneralizedNewtypeDeriving, CPP, ApplicativeDo #-}
module Database.PostgreSQL.Simple.Options where
import Database.PostgreSQL.Simple
import Options.Applicative
import Text.Read
import Data.ByteString (ByteString)
import qualified Data.ByteString.Char8 as BSC
import GHC.Generics
import Options.Generic
import Data.Typeable
import Data.String
import Data.Monoid
import Data.Either.Validation
import Data.Default

The "Partial" Option Types

In general, options types are built from many optional fields. Additionally, multiple options sets can be combined (i.e. command line options, config file, environment vars, defaults, etc). The easiest way to handle this is to create a "partial" option family that can be monoidally composed and is "completed" with a default option value.

-- | An optional version of 'Options'. This includes an instance of
-- | 'ParseRecord' which provides the optparse-applicative Parser.
data PartialOptions = PartialOptions
  { host     :: Last String
  , port     :: Last Int
  , user     :: Last String
  , password :: Last String
  , database :: Last String
  } deriving (Show, Eq, Read, Ord, Generic, Typeable)

We will utilize a boilerplate prevention library by Gabriel Gonzalez called optparse-generic which generates a parser from the record field names.

To create the parser we have to merely declare an instance of ParseRecord.

instance ParseRecord PartialOptions

Now we make PartialOptions an instance of Monoid so we can combine multiple options together.

instance Monoid PartialOptions where
  mempty = PartialOptions (Last Nothing) (Last Nothing)
                              (Last Nothing) (Last Nothing)
                              (Last Nothing)
  mappend x y = PartialOptions
    { host     = host     x <> host     y
    , port     = port     x <> port     y
    , user     = user     x <> user     y
    , password = password x <> password y
    , database = database x <> database y
    }

As it so happens there are two ways to create a db connection with postgresql-simple: Options and a ByteString connection string. We have a partial version of Options but we need something for the connection string.

newtype ConnectString = ConnectString
  { connectString :: ByteString
  } deriving ( Show, Eq, Read, Ord, Generic, Typeable, IsString )

I don't like the default option parsing for String in optparse-applicative. I want something that will escape double quotes, remove single quotes or just use the string unaltered. The function parseString does this.

unSingleQuote :: String -> Maybe String
unSingleQuote (x : xs@(_ : _))
  | x == '\'' && last xs == '\'' = Just $ init xs
  | otherwise                    = Nothing
unSingleQuote _                  = Nothing

parseString :: String -> Maybe String
parseString x = readMaybe x <|> unSingleQuote x <|> Just x

We use parseString to make a custom instance of ParseRecord.

instance ParseRecord ConnectString where
  parseRecord =  fmap (ConnectString . BSC.pack)
              $  option ( eitherReader
                        $ maybe (Left "Impossible!") Right
                        . parseString
                        )
                        (long "connectString")

Thus, my PartialOptions type is either the ConnectString or the PartialOptions type.

data PartialOptions
  = POConnectString      ConnectString
  | POPartialOptions PartialOptions
  deriving (Show, Eq, Read, Generic, Typeable)

instance Monoid PartialOptions where
    mempty = POPartialOptions mempty
    mappend a b = case (a, b) of
        (POConnectString x, _) -> POConnectString x
        (POPartialOptions x, POPartialOptions y) ->
            POPartialOptions $ x <> y
        (POPartialOptions _, POConnectString x) -> POConnectString x

There is one wrinkle. optparse-generic treats sum types as "commands". This makes sense as a default, but it is not what we want. We want to choose one record or another based on the non-overlapping flags. This is easy enough to do by hand.

instance ParseRecord PartialOptions where
  parseRecord
    =  fmap POConnectString      parseRecord
   <|> fmap POPartialOptions parseRecord

The Composable Parser

We can use PartialOptions as the type of a field in a larger options record defined elsewhere. When defining this more complicated parser, we reuse the work we did here by calling parseRecord. To make it even clearer we create an alias called parser so clients will know what to use.

-- | The main parser to reuse.
parser :: Parser PartialOptions
parser = parseRecord

The Complete Option

The connection option for postgresql-simple is either the record Options or a connection string

data Options
  = OConnectString ByteString
  | OOptions   Options
  deriving (Show, Eq, Read, Generic, Typeable)

Option "completion"

postgresql-simple provides sensible defaults for Options via defaultOptions. We use these as the defaults when parsing. We create a PartialOptions with these defaults.

mkLast :: a -> Last a
mkLast = Last . Just

-- | The 'PartialOptions' version of 'defaultOptions'
instance Default PartialOptions where
    def = PartialOptions
      { host     = mkLast $                connectHost     defaultOptions
      , port     = mkLast $ fromIntegral $ connectPort     defaultOptions
      , user     = mkLast $                connectUser     defaultOptions
      , password = mkLast $                connectPassword defaultOptions
      , database = mkLast $                connectDatabase defaultOptions
      }

instance Default PartialOptions where
    def = POPartialOptions def

We can now complete the PartialOptions to get a Options.

getOption :: String -> Last a -> Validation [String] a
getOption optionName = \case
    Last (Just x) -> pure x
    Last Nothing  -> Data.Either.Validation.Failure
        ["Missing " ++ optionName ++ " option"]

completeOptions :: PartialOptions -> Either [String] Options
completeOptions PartialOptions {..} = validationToEither $ do
  connectHost     <- getOption "host"     host
  connectPort     <- fromIntegral
                 <$> getOption "port"     port
  connectUser     <- getOption "user"     user
  connectPassword <- getOption "password" password
  connectDatabase <- getOption "database" database
  return $ Options {..}

Completing a PartialOptions to get an Options follows straightforwardly ... if you've done this a bunch I suppose.

-- | mappend with 'defaultPartialOptions' if necessary to create all
--   options
completeOptions :: PartialOptions -> Either [String] Options
completeOptions = \case
  POConnectString   (ConnectString x) -> Right $ OConnectString x
  POPartialOptions x              -> OOptions <$> completeOptions x

The Option Parser

Parse a PartialOptions and then complete it. This is not composable but is convient for testing and if you only need a Option type

-- | Useful for testing or if only Options are needed.
completeParser :: Parser Options
completeParser =
    fmap (either (error . unlines) id . completeOptions . mappend def) parseRecord

The Runner

As a convenience, we export the primary use of parsing connection options ... making a connection.

-- | Create a connection with an 'Option'
run :: Options -> IO Connection
run = \case
  OConnectString connString -> connectPostgreSQL connString
  OOptions   connInfo   -> connect           connInfo

The tests

Testing is pretty straightforward using System.Environment.withArgs. See the Spec.hs for examples of how to test the parsers.