Tutorial

Tutorial

Suppose we are writing a simple program that creates new user accounts - we’ll call it “mkuser”. The goal will be to provide a command line interface with the following syntax:

mkuser [--uid=INT] [--system] [--groups={GROUP...}]
  [--quota=[soft=INT],[hard=INT]] USERNAME

This file is a literate Haskell program. That means it also is a valid Haskell program that you can compile:

$ stack exec ghc -- -o mkuser doc/MkUser.lhs
[1 of 2] Compiling Main             ( doc/MkUser.lhs, doc/MkUser.o )
[2 of 2] Linking mkuser

Getting Started

First of all, we will need to write lots of NonEmpty list and Text literals, so we’ll enable some language extensions to make that easier.

{-# LANGUAGE OverloadedLists   #-}
{-# LANGUAGE OverloadedStrings #-}

Now let’s import the modules we need. The building blocks for our parser are found in Mangrove.Unix while the functions for running parsers live in the Mangrove module. We’ll use combinators from Control.Applicative to construct our parser. We’ll also import Text, since Mangrove is built around Text rather than String. Finally, we’ll use Data.Version when specifying program metadata.

import           Control.Applicative
import           Data.Text           (Text)
import           Mangrove
import           Mangrove.Unix
import           Data.Version

Next let’s create a new record that captures the program’s runtime configuration:

-- | This record encapsulates all our programs runtime options.
data Settings = Settings
  { userId     :: Maybe Int -- ^ An optional target user ID
  , userSystem :: Bool -- ^ Is this a system user?
  , userGroups :: [Text] -- ^ Groups the new user will be in
  , userQuota  :: Maybe Quota -- ^ Disk usage quotas
  , userName   :: Text  -- ^ Username for the new user
  } deriving (Show)

-- | This record represents disk usage quota settings for a new user.
data Quota = Quota
  { quotaSoft :: Maybe Int -- ^ Limit new data after this point
  , quotaHard :: Int -- ^ Hard upper limit on disk usage
  } deriving (Show)

Let’s also pretend that our program’s logic lives inside a function run :: Settings -> IO (). We pass it the settings we want, and it runs the program accordingly. However, since this is just an example program, we won’t actually create any user accounts; instead we’ll just have the program print its settings to stdout.

run :: Settings -> IO ()
run = print

Parser Types

We want to construct a parser that reads a list of arguments and yields a Settings. That means our parser will have the type UnixParser Settings.

UnixParser is actually a type synonym for ParseTree UnixScheme, where UnixScheme is a parsing scheme for UNIX-style command line syntax. Alternative parsing schemes can be defined, but that’s beyond the scope of this example.

Now, let’s examine the building blocks of parsers. We’ll start with the most basic components and work our way toward more complicated parsers.

Text Parsers

TextParsers are the most basic type of parser in Mangrove. Here’s the definition for reference:

data TextParser r = TextParser
  { parserHint :: !Text
  , parserRun  :: Text -> Either Text r
  }

We can see this is just a wrapper around a simple text parsing function, with the addition of a “hint”. The hint is just a short descriptive string we can use to represent an input when displaying usage information. It’s traditional to use a single word in all-caps for this, like PATH, STRING, or INPUT_FILE.

Mangrove provides text parsers for common types like Int, Double, Text, and String in the Mangrove.TextParser module. Additionally, instead of manually specifying a parser for each type, we can rely on Haskell’s type system to choose one automatically. The defaultParser function provides this capability: it’s a polymorphic parser for any type that has a DefaultParser instance (such as the aforementioned common types). Thus, when defaultParser is used in a context where the type known, Haskell automatically selects the appropriate parser for that type.

Positional Parameters

A “positional parameter” is a positional input that accepts the first non-flag argument it encounters. In Mangrove, positional parameters are usually just referred to as “parameters” since other kinds of parameters have their own names.

Consider an example program called substring whose command line syntax is substring START END STRING. START, END, and STRING would be parameters. If we invoke substring 1 3 "example", we know that START is 1, END is 3, and STRING is "example" because of the order in which they appear.

Our mkuser program only needs one positional parameter: a username. We define a parser for it using the parameter function, which converts a TextParser into a positional parameter parser. Because a DefaultParser instance exists for strict Text, we use defaultParser; however, we override the parser’s hint (normally "STRING") with "USERNAME" which is more descriptive.

prm_name :: UnixParser Text
prm_name = parameter (defaultParser {parserHint = "USERNAME"})

Options

An “option” is a named input. It consists of a flag (like --uid or -u) followed by an optional subargument string. Per UNIX tradition, long flags use double dashes and short flags use a single dash. For long flags, the subargument can be separated by an equal sign (--uid=1000) or a space (--uid 1000). Short flags can be directly concatenated with their subargument (-u1000) or separated by a space (-u 1000).

Let’s define a parser for the --uid option, which allows the user to specify a user ID for the new user if they want. It should accept one subparameter, an integer UID.

opt_uid :: UnixParser Int
opt_uid = option ["--uid", "-u"]
          "Specify a user ID"
          $ subparameter defaultParser

Switches

The --system option is simpler because it doesn’t accept any subarguments - it is either present (True) or absent (False). This special type of option is a “switch”, and we can use the switch function to create a parser:

opt_system :: UnixParser Bool
opt_system = switch ["--system", "-s"] "Create a system user"

If we defined this without ‘switch’ it would look like this:

opt_system = option ["--system", "-s"]
             "Create a system user"
             (pure True)
             <|> pure False

Options with Multiple Subparameters

Now that we’ve seen options with no subarguments, and options with a single subargument, let’s try defining an option that can accept multiple subarguments.

The --groups option is a bit different from the --uid option because we want the user to be able to specify a list of groups for the new user to join. Thus, we want to create an option that accepts one or more subarguments.

Thankfully, SubParser is also an Alternative instance. We can use some (from Control.Applicative) to convert a SubParser r into a SubParser [r] that will expect to parse one or more r values.

opt_groups :: UnixParser [Text]
opt_groups =
  option ["--groups", "-g"]
  "Specify what groups the user is part of"
  $ some $ subparameter defaultParser {parserHint = "GROUP"}

Mangrove recognizes that the subparser some $ subparameter defaultParser :: SubParser [Text] can consume multiple subarguments, so it automatically splits the subarguments by comma. This allows us to pass a list of group names like so: --groups=wheel,audio,input, and the parser will yield ["wheel","audio","input"].

By using some instead of the similar function many, we have created a subparser that will fail if no subarguments are provided (e.g. mkuser alice --groups).

Suboptions

So far, we’ve seen options that accept any number of subparameters. But what if we need an option that accepts structured data - not just a list of values, but key/value pairs? This is common in configuration and deployment tools. For example, a mount command might need to specify both a source and destination path, along with optional flags. Rather than forcing the user to remember exact positional order, suboptions use named pairs: --mount src=/webroot,dst=/var/www,ro.

A suboption is a named key/value pair within a single option’s arguments. Unlike positional subarguments (which are parsed in order), suboptions are identified by name, so they can appear in any order. Suboptions are separated by commas (just like positional subarguments), but each suboption has the syntax key=value. You can also mix suboptions with positional subarguments in the same option, and Mangrove will parse all of them together.

When Mangrove begins parsing a list of subarguments for some option, it checks whether the parse tree contains any suboption parsers. If it does, Mangrove treats any subarguments that contain an = sign as key/value pairs rather than positional subarguments. This mechanism prevents confusion between the two parsing modes and ensures that suboptions and subparsers don’t interfere with each other. If the parser subtree contains no suboption parsers at all, Mangrove ignores the = sign and treats all subarguments as positional.

Like regular options, suboptions can be optional or required. You can use the same Applicative combinators to express this: optional makes a suboption optional (returning Nothing if absent), and <|> allows you to provide a default value.

Let’s add a --quota option to our mkuser program for configuring a user’s disk usage quota. --quota will accept two suboptions: soft and hard, for soft and hard space limits respectively. Each suboption is itself optional and accepts a single integer value.

opt_quota :: UnixParser Quota
opt_quota =
  option ["--quota"]
  "Set a disk usage quota"
  $ Quota <$> subopt_soft
          <*> subopt_hard
  where
    subopt_soft :: SubParser (Maybe Int)
    subopt_soft = optional $ suboption "soft" defaultParser

subopt_hard :: SubParser Int
subopt_hard = suboption "hard" defaultParser <|> pure 512

Applicative

You might notice that opt_uid has the type UnixParser Int, but our Settings record expects Maybe Int. That’s because we need to handle the case where no --uid option appears in the arguments. The solution is to wrap the parser using optional from Control.Applicative: optional opt_uid attempts to parse the --uid option, returning Nothing if it’s absent.

For --groups, we want slightly different behavior: if the user doesn’t provide --groups, we should use an empty list instead of returning Nothing. We can express this using the <|> operator (i.e. the alternative operator): opt_groups <|> pure [] means “try parsing --groups, and if that fails, just return an empty list.”

Now we can finally construct our Settings parser using Applicative notation:

parseSettings :: UnixParser Settings
parseSettings =
  Settings
  <$> optional opt_uid
  <*> opt_system
  <*> (opt_groups <|> pure [])
  <*> optional opt_quota
  <*> prm_name

Request Options

The CLI interface we’ve defined is missing something important: an option for displaying help and usage information. Let’s create a new Settings parser that recognizes --help as a request for help information. While we’re at it, we can also add a --version option that displays the program’s version.

Some options, like --help and --version, don’t produce values that go into our Settings record. Instead, they interrupt parsing to provide information (help text or version number) and exit. Mangrove calls these “request options”. When a request option is matched, parsing stops immediately and Mangrove prints the requested information without trying to fill in the rest of Settings.

There are currently 2 kinds of requests: HelpRequests and VersionRequests. A help request is a request for help and usage information for our parser. A version request is for querying the program’s version.

parseSettings' :: UnixParser Settings
parseSettings' = opt_help <|> opt_version <|> parseSettings
  where
    opt_help =
      requestOption ["--help"]
      "Display help and usage information"
      HelpRequest
    opt_version =
      requestOption ["--version"]
      "Display program version"
      VersionRequest

Running the Parser

In order to display help or version information, Mangrove needs a few details about the program. This metadata is passed in using a ProgramInfo record:

-- Here the type variable @s@ is a phantom type that we can leave
-- undetermined.
programInfo :: ProgramInfo s
programInfo = ProgramInfo
  { programName = "mkuser" -- The name of the program
  , programVersion = makeVersion [0,1,2,3] -- The program version is "0.1.2.3"
  , programDesc = "Create user accounts" -- A short description of the program
  }

The parseArguments function will run our parser with the arguments passed to our program by the operating system.

main :: IO ()
main = parseArguments programInfo parseSettings' run

parseArguments takes three arguments: the program metadata, a parser, and a function of type r -> IO a. When the parser completes successfully, this function will be called with the result. If parsing fails, an error message will be printed to stderr and the program will exit indicating failure. If a request for help or version information is triggered, the program will respond by printing the requested information to stdout and then exiting normally.

If you want to run an argument parser without using IO, or you want to pass your own argument list, check out runHelpfulParser from Mangrove.

Now we have a complete program we can build and run to show the argument parser in action! Here’s how the completed program behaves with different inputs:

$ ./mkuser --system --groups audio,input bilbo
Settings {userId = Nothing, userSystem = True, userGroups =
["audio","input"], userQuota = Nothing, userName = "bilbo"}

$ ./mkuser --quota soft=500,hard=600 frodo
Settings {userId = Nothing, userSystem = False, userGroups = [],
userQuota = Just (Quota {quotaSoft = Just 500, quotaHard = 600}),
userName = "frodo"}

$ ./mkuser --badinput
unexpected --badinput

$ ./mkuser --system
expected: USERNAME

$ ./mkuser --uid=InvalidNumber bilbo
--uid=InvalidNumber: InvalidNumber: input does not start with a digit