Haskell library that supports command-line flag processing https://github.com/josercruz01/hsoptions

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Apache-2.0 licensed by Jose Raymundo Cruz


HsOptions is a Haskell library that supports command-line flag processing.

It is equivalent to getOpt(), but for Haskell, and with a lot of neat extra features. Typically, an application specifies what flags it is expecting from the user -- like --user_id or -file <filepath> -- somehow in the code, HsOptions provides a declarative way to define the flags in the code by using the make function.

Most flag processing libraries requires all the flags to be defined in a single point, such as the main file, but HsOptions allows the flags to be scattered around the code, promoting code reuse and scalability. A module defines the flags it needs and when this module is used in other modules it's flags are handled by HsOptions.

HsOptions is completely functional, specially because no global state is modified. The only IO actions performed are to get the command-line arguments and to expand the configuration files.

Another important feature of HsOptions is that it can process flags from text files as well as from command-line. This feature is available with the use of the special --usingFile <filename> flag.

For example:

# inside 'file1.conf'
--user_name batman

... when running the Program.hs haskell program:

$ runhaskell Program.hs --debug --usingFile file1.conf -f

=== is evaluates the same as ==== >

$ runhaskell Program.hs --debug --user_name batman --pretty -f

Each configuration file is expanded after it is processed, so it can include more configuration files and create a tree. This is useful to create different environments, like production.conf, dev.conf and qa.conf just to name a few.

Build Status

Table of contents


The library depends on cabal (Install Cabal).

To install using cabal:

cabal install hsoptions


See Examples for more examples.

This program defines two flags (user_name of type String and age of type Int) and in the main function prints the name and the age plus 5. It also adds the alias u to the flag user_name.

-- Program.hs
import System.Console.HsOptions

userName = make ( "user_name"
                , "the user name of the app"
                , [ parser stringParser
                  , aliasIs ["u"]
userAge = make ("age", "the age of the user", [parser intParser])

flagData = combine [flagToData userName, flagToData userAge]

main :: IO ()
main = processMain "Simple example for HsOptions."

success :: ProcessResults -> IO ()
success (flags, args) = do let nextAge = (flags `get` userAge) + 5
                           putStrLn ("Hello " ++ flags `get` userName)
                           putStrLn ("In 5 years you will be " ++
                                     show nextAge ++
                                     " years old!")

failure :: [FlagError] -> IO ()
failure errs = do putStrLn "Some errors occurred:"
                  mapM_ print errs

You can run this program in several ways:

$ runhaskell Program.hs --user_name batman --age 23
Hello batman
In 5 years you will be 28 years old!

... or:

$ runhaskell Program.hs --user_name batman --age ten
Some errors occurred:
Error with flag '--age': Value 'ten' is not valid

... or:

$ runhaskell Program.hs --help
Simple example for HsOptions.
    --age        the age of the user
-u  --user_name  the user name of the app
    --usingFile  read flags from configuration file
-h  --help       show this help


Defining flags

A flag is defined using the make function. It takes the name of the flag, the help text and the parser. The parser specified how to parse the string value of the flag to the correct type. A set of default parsers are provided in the library for common types.

To define a flag of type Int:

age :: Flag Int
age = make ("age", "age of the user", [parser intParser])

To define the same flag of type Maybe Int:

age :: Flag (Maybe Int)
age = make ("age", "age of the user", [maybeParser intParser])

The function maybeParser is a wrapper for a parser of any type that converts that parser to a Maybe data type, allowing the value to be Nothing. This is used mostly for optional flags.

Instead of intParser the user can specify his custom function to parse the string value to the corresponding flag type. This is useful to allow the user to create flags of any custom type.

Process flags

To process the flags the processMain function is used. This function serves as a middle man between the real main and the flag processing. Takes 5 arguments:

  • The description of the program: used when printing the help text.
  • A collection of all the defined flags
  • Three callback functions:
    • Success callback: called with the process results if no errors occurred
    • Failure callback: called if any error while processing flags occurred
    • Display help callback: called if the user sent the --help flag

This is an example on how to call the processMain function:

import System.Console.HsOptions

-- flags definitions
name = make ("name", "the name of the user", [parser stringParser])
age = make ("age", "the age of the user", [parser intParser])

-- collection of all flags
all_flags = combine [flagToData age, flagToData name]

-- real main
main = processMain "Example program for processMain"

-- new main function
successMain (flags, args) = putStrLn $ flags `get` name

In this example, the provided implementations for the failure and the display help callback were used (defaultDisplayErrors and defaultDisplayHelp), so that we do not need to define how to print errors or how to print help.

As mentioned before, if no errors were found then successMain function is called. The argument sent is a tuple (FlagResults, ArgsResults). FlagResults is a data structure that can be used to get the flag's value with the get function. ArgResults is just a list of the non-flag positional arguments.

If there was any kind of errors while processing the flags the display errors callback argument is called with the list of FlagError as argument. The user can specify a custom function so he handles the argument as he wishes.

The third callback, display help, is called when the user sent the special help flag (--help or -h). It takes the program description and all the information of the flags as a list of (flag_name, [flag_alias], flag_helptext). The defaultDisplayHelp is a default implementation that prints the helptext in a standard format, usually this is the way to go unless the user wants to print the help text in a custom format.

Get flag value

A flag value is obtained by using the get function. It takes the FlagResults and a defined flag as a parameter, and it will look for the value of the flag inside the FlagResults. In a way you can think of FlagResults as a data structure that can be queried with flags to retrieve flag values.

The FlagResults are obtained by processing the flags with the processMain function.

The return type of get is the type of the flag, so if the flag is Flag Int then get returns an Int (so the flag value is typed).

For a given flag:

repeat = make ("repeat", "how many times to repeat", [parser intParser])

... we can grab it's value after processing like this:

success :: (FlagResults, ArgsResults) -> IO ()
success (flags, args) = do let r = flags `get` repeat
                           putStrLn $ "The value of repeat is " ++ show r

Optional and Required flags

By default all flags are marked as required. If you want to make an optional flag then two things are required:

* First, the type of the flag must be `Flag (Maybe a)`, so that the flag can
be `Nothing` if it was not provided and `Just value` if it was.

* Second, the flag must be configured using the `isOptional` flag


-- optional flag
database :: Flag (Maybe String)
database = make ("db", "the database", [maybeParser stringParser, isOptional])

-- required flag
app_id :: Flag Int
app_id = make ("app_id", "application to run", [parser intParser])

-- combine all flags
all_flags = combine [flagToData database, flagToData app_id]

-- main
main = processMain "Sample" all_flags success
                   defaultDisplayErrors defaultDisplayHelp

-- success main
success (flags, _) = do putStrLn $ "database: " ++ show (flags `get` database)
                        putStrLn $ "app_id: " ++ show (flags `get` app_id)

This is the expected behavior when getting the flag value:

$ runhaskell Program.hs
Errors occurred while parsing flags:
Error with flag '--app_id': Flag is required

... as you can see only app_id is required, but not database.

$ runhaskell Program.hs --app_id = 123
database: Nothing
app_id: 123

... value for database is Nothing.

$ runhaskell Program.hs --app_id = 123 --db = local
database: Just "local"
app_id: 123

Configuration files

Flags can be processed not only from command-line input, but also from configuration text files. These text files are included at any point in the command-line stream by using the special flag --usingFile <filename>.

When the flag processor encounters a usingFile it reads the content of the file and runs the processor again with this content, consuming the usingFile flag and replacing it with all the new flags found inside the configuration file.

A configuration file can itself include other configuration files as well, by using the usingFile flag inside the file, so a tree of files can be created (a file can have a parent file, and a grandparent file, or a file can include multiple files to combine them together).

If there is any kind of error while reading the file, or there is a syntax error inside the file then that error is reported to the user.

This is an example of a configuration file that has comments, and that includes two more files.

# combined.conf
--database = localdb
--usingFile = file1.conf
--usingFile = file2.conf
# file1.conf
--flagA = 3
# file2.conf
--flagB = 42

So if we have a Program.hs that is configured with the flags database, flagA and flagB, and that prints the remaining positional arguments, then this is the output of the program for the following scenarios:

$ runhaskell Program.hs --usingFile combined.conf
database: localdb
flagA: 3
flagB: 42
args: ["jack","jill","batman"]

We can send more arguments, or modify flags, after or before including the file:

$ runhaskell Program.hs superman --usingFile combined.conf robin
database: localdb
flagA: 3
flagB: 42
args: ["superman", "jack","jill","batman", "robin"]

... as you can observe superman and robin are respectively at the start and end of the positional arguments, that is because first superman is found in the input stream, then the usingFile combined.conf which gets evaluated and parsed, and when this is complete then the processor moves to robin which is captured as the last positional argument.

Here is another example on how we can override and extend the flags. We will change the flagA to 1024 and will append the value .local to the database flag.

$ runhaskell Program.hs --usingFile combined.conf --database +=! ".local" --flagA = 1024
database: localdb.local
flagA: 1024
flagB: 42
args: ["jack","jill","batman"]

Default value

There is two types of default flag values, a default value when the flag was not provided by the user, and another default value for when the user provided the flag but not the flag value. The flag configurations are defaultIs and emptyValueIs.

A default value can be configured for a flag by using the defaultIs flag configuration. It takes the value that the flag will have in case the flag is not provided by the user.


database = make ("database", "the db connection", [ parser stringParser
                                                  , defaultIs "local.sqlite"])

So for example:

    $ runhaskell Program.hs
    database: local.sqlite

... if you set the value then the default is ignored:

    $ runhaskell Program.hs --database production.sqlite
    database: production.sqlite

... but, it should be noted that if you send the flag, but not it's value, then an error will occur, as the system assumes you meant to set a value to the flag:

    $ runhaskell Program.hs --database
    Some errors occurred:
    Error with flag '--database': Flag value was not provided

... if you want to add a default value for the flag value is empty use the emptyValueIs flag configuration:

database = make ("database", "the db connection", [ parser stringParser
                                                  , defaultIs "local.sqlite",
                                                  , emptyValueIs "prod.sqlite"])
    $ runhaskell Program.hs --database
    database: prod.sqlite

The combination of defaultIs and emptyValueIs makes it possible to define flags such as booleans. So we could set up a flag such as --debug (Bool) that will take the value False if missing and will take the value True if the user sent --debug without him having to say --debug = True.

Common configurations

There are some common patterns that occurs while configuring flags. These patterns can be put into a function for code reuse.

Boolean flag

A default behavior for boolean flag is that if the flag is missing then it's value is False and if the flag is present, even with a missing flag value, then it's value is True.

For this the boolFlag flag configuration was created.

debug = make ("debug", "debug flag", boolFlag])

This is equivalent to:

debug = make ("debug", "debug flag", [ parser boolParser
                                     , defaultIs False
                                     , emptyValueIs True

This is because boolFlag is defined as such:

boolFlag :: [FlagConf Bool]
boolFlag = [ parser boolParser
           , defaultIs False
           , emptyValueIs True

Flag alias

Creates a flag configuration for the aliases of the flag.

Sets multiple alias for a single flag. (--user_id alias: ["u", "uid"). These aliases can be used to set the flag value, so --user_id = 8 is equivalent to -u = 8.

They are set using the aliasIs flag configuration:

user_id = make ("user_id", "the id", [parser intParser, aliasIs ["u", "uid"]])

Dependent defaults

Creates a flag configuration that will define a default value for a flag based on a condition. This condition is a function that takes in the current FlagResults and returns Nothing if the there is no default value or the default value (Just) if there is one.

If the function returns a value, and the user did not send the flag in the input stream, then the default value associated with this function is used as the default value for the flag.

The dependent default value is configured by using the defaultIf function. It takes as arguments the default value getter function that given the FlagResults tries to return a default value.


userName = make ("user_name", "the user", [parser stringParser])

movie = make ( "movie"
             , "the movie of the user"
             , [ parser stringParser
               , defaultIf (\ flags ->
                     if flags `get` userName == "neo"
                     then Just "matrix"
                     else if flags `get` userName == "bruce"
                          then Just "batman"
                          else Nothing)

This is the output for different scenarios:

    $ runhaskell Program.hs --user_name other
    Some errors occurred:
    Error with flag '--movie': Flag is required

... since non of the predicate matched then the flag is required to the user.

    $ runhaskell Program.hs --user_name batman
    user_name: bruce
    movie: batman-begins

... as you can see the first dependent default matched, so it's value is used.

    $ runhaskell Program.hs --user_name neo
    user_name: neo
    movie: batman-matrix

This configuration is useful in scenarios where a flag's default value depends on the value of on or more flags.

Optionally required

You can mark a flag optionally required by using the requiredIf flag configuration.

This flag configuration needs a predicate function that given the current FlagResults returns True or False depending if the flag should or should not be required.

For example it is useful to make a flag required if another flag was set to a particular value:

log_memory = make ( "log_memory"
                  , "if set to true the memory usage will be logged"
                  , boolFlag)

log_output = make ( "log_output"
                  , "where to save the log. required if 'log_memory' is true"
                  , [ maybeParser stringParser
                    , requiredIf (\ flags -> flags `get` log_memory == True)

... after the flags are processed then the optionally required condition is checked. If the configured predicate returns true an error is reported to the user:

    $ runhaskell Program.hs
    log_memory: False
    log_output: Nothing

... if you send the log_memory the conditional predicate will return True and the flag will be required:

    $ runhaskell Program.hs  --log_memory
    Some errors occurred:
    Error with flag '--log_output': Flag is required

... if you send the value for log_output then an error should not occur:

    $ runhaskell Program.hs  --log_memory --log_output /tmp/memorylog.tmp
    log_memory: True
    log_output: Just "/tmp/memorylog.tmp"

Global validation

A global validation rule is a function that will be evaluated with the FlagResults after the processing stage and will determine if the current state is valid.

It is the last stage of flag processing. If there is a validation error then this error is reported to the user. This validation is done by using the validate function that takes a function that returns a Maybe String, Nothing being a passing result and Just err being failing result with an err error message.

For example:

flagData = combine [ flagToData user_id
                   , validate (\fr -> if get fr user_id < 0
                                      then Just "user id negative error"
                                      else Nothing)

An error will be produces if the application is run with a negative user_id.

Flag parsers

Flag parser configurations.



Parses a flag value to an integer.


Parses a flag value to a float.


Parses a flag value to a double.


Parses a flag value to a char.


Parses a flag value to a string.


Parses a flag value to a boolean.


Parses a flag value to an array.

Parser wrappers


Takes a parser as argument and wraps it so it becomes a Maybe a parser.

Used to convert an existent parser to an optional parser.

intPaser :: FlagArgument -> Int
toMaybeParser intParser :: FlagArgument -> Maybe Int

If the flag was missing or the flag value was missing then the new parser will return Nothing, otherwise the wrapped parser is called.

It comes handy when you create a flag of type Maybe a and you want to use one of the existent parsers:

user_id :: Flag (Maybe Int)
user_id = make ("user_id", "help", [parser (toMaybeParser intParser)])

Since this seems to be a common pattern the maybeParser method was created that combines the parser function with the toMaybeParser. The previous example is equivalent to:

user_id :: Flag (Maybe Int)
user_id = make ("user_id", "help", [maybeParser intParser])

Flag operations

Flag operations allows the user to set the value of a flag based on the previous value set. This is useful in situations where configuration files are used, so that a child configuration file can extend the value of a flag set in a parent configuration file.

Operations are specified when setting a value for a flag. This is the syntax to set a flag: --flag_name [operation] flag_value. If the [operation] is not set then the assign (=) operation is implied.


This is the default operation. Sets the value of the flag, overwriting any previous value if there was any. This is the default operation unless the user changed it in the flag configuration.


    $ runhaskell Program.hs --file = "/home/user/" --file = "/tmp"
    file: "/tmp"

Inherit keyword

The $(inherit) keyword can be used in the flag value and will be expanded to the previous value of the flag (or to empty string if no previous value).


    $ runhaskell Program.hs --file = "/home/user" --file = "$(inherit)/local/tmp"
    file: "/home/user/local/tmp"

... and with no previous value:

    $ runhaskell Program.hs --file = "$(inherit)/local/tmp"
    file: "/local/tmp"


It's an specification of the $(inherit) keyword to append the current value of the flag to the previous. There are two ways to append, using the += symbol or the +=! symbol.

They are the same except that += puts a space between previous value and current value (if there is a previous value for the flag).

They are equivalent to:

    --file += /local/tmp   <=> --file = "$(inherit) /local/tmp"   -- space in between
    --file +=! /local/tmp  <=> --file = "$(inherit)/local/tmp"    -- no space in between

Example (+=):

    $ runhaskell Program.hs --warning = "1 2" --warning += "3"
    warning: "1 2 3"

Example (+=!):

    $ runhaskell Program.hs --warning = "warn-1,2" --warning +=! ",3"
    warning: "warn-1,2,3"


It's an specification of the $(inherit) keyword to prepend the current value of the flag to the previous. There are two ways to prepend, using the =+ symbol or the =+! symbol.

They are the same except that =+ puts a space between previous value and current value (if there is a previous value for the flag).

They are equivalent to:

    --file =+ /local/tmp   <=> --file = "/local/tmp $(inherit)"   -- space in between
    --file =+! /local/tmp  <=> --file = "/local/tmp$(inherit)"    -- no space in between

Example (=+):

    $ runhaskell Program.hs --warning = "1 2" --warning =+ "0"
    warning: "0 1 2"

Example (=+!):

    $ runhaskell Program.hs --warning = "warn-1,warn-2" --warning =+! "warn-0,"
    warning: "warn-0,warn-1,warn-2"

Change flag default operation

By default a flag's default operation is the assign (=) operation. So if the user sends a flag and it's value without explicitly using an operation this is the operation used.

Now if you want to change this behavior for a given flag you can do so by using the operation flag configuration. This takes an operation as an argument and sets this as the default operation for the flag:

warning = make ("warn", "warnings to print", [parser stringParser, operation append])

Now if you run the program like this:

    $ runhaskell Program.hs --warn 1 --warn 2 --warn 3
    warn: "1 2 3"

You can overwrite this default if you specify the operation in the command line:

    $ runhaskell Program.hs --warn 1 --warn 2 --warn 3 --warn = 0
    warn: "0"

The available operations for the flag are these:

    * `assign` (=)
    * `append` (+=)
    * `append'` (+=!)
    * `prepend` (=+)
    * `prepend'` (=+!)


Build from source using build (build and run tests):

$ ./build

Or using cabal:

$ cabal build     -- builds the text
$ cabal test      -- runs all tests
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