Cabal- A framework for packaging Haskell software

CopyrightDuncan Coutts 2007
Portabilitynon-portable (ExistentialQuantification)
Safe HaskellNone




This is to do with command line handling. The Cabal command line is organised into a number of named sub-commands (much like darcs). The CommandUI abstraction represents one of these sub-commands, with a name, description, a set of flags. Commands can be associated with actions and run. It handles some common stuff automatically, like the --help and command line completion flags. It is designed to allow other tools make derived commands. This feature is used heavily in cabal-install.


Command interface

data CommandUI flags




commandName :: String

The name of the command as it would be entered on the command line. For example "build".

commandSynopsis :: String

A short, one line description of the command to use in help texts.

commandUsage :: String -> String

A function that maps a program name to a usage summary for this command.

commandDescription :: Maybe (String -> String)

Additional explanation of the command to use in help texts.

commandNotes :: Maybe (String -> String)

Post-Usage notes and examples in help texts

commandDefaultFlags :: flags

Initial / empty flags

commandOptions :: ShowOrParseArgs -> [OptionField flags]

All the Option fields for this command

commandShowOptions :: CommandUI flags -> flags -> [String]

Show flags in the standard long option command line format

data CommandParse flags


CommandHelp (String -> String) 
CommandList [String] 
CommandErrors [String] 
CommandReadyToGo flags 


Functor CommandParse 



:: CommandUI flags 
-> Bool

Is the command a global or subcommand?

-> [String] 
-> CommandParse (flags -> flags, [String]) 

Parse a bunch of command line arguments

getNormalCommandDescriptions :: [Command action] -> [(String, String)]

Helper function for creating globalCommand description

Constructing commands

usageDefault :: String -> String -> String

Default "usage" documentation text for commands.

usageAlternatives :: String -> [String] -> String -> String

Create "usage" documentation from a list of parameter configurations.



:: String


-> String


-> [String]

usage alternatives

-> flags

initial/empty flags

-> (ShowOrParseArgs -> [OptionField flags])


-> CommandUI flags 

Make a Command from standard GetOpt options.

hiddenCommand :: Command action -> Command action

Mark command as hidden. Hidden commands don't show up in the 'progname help' or 'progname --help' output.

Associating actions with commands

data Command action

commandAddAction :: CommandUI flags -> (flags -> [String] -> action) -> Command action

noExtraFlags :: [String] -> IO ()

Utility function, many commands do not accept additional flags. This action fails with a helpful error message if the user supplies any extra.

Building lists of commands

data CommandSpec action

wraps a CommandUI together with a function that turns it into a Command. By hiding the type of flags for the UI allows construction of a list of all UIs at the top level of the program. That list can then be used for generation of manual page as well as for executing the selected command.


forall flags . CommandSpec (CommandUI flags) (CommandUI flags -> Command action) CommandType 

Running commands

commandsRun :: CommandUI a -> [Command action] -> [String] -> CommandParse (a, CommandParse action)

Option Fields

data OptionField a

We usually have a data type for storing configuration values, where every field stores a configuration option, and the user sets the value either via command line flags or a configuration file. An individual OptionField models such a field, and we usually build a list of options associated to a configuration data type.




optionName :: Name
optionDescr :: [OptDescr a]

type Name = String

Constructing Option Fields

option :: SFlags -> LFlags -> Description -> get -> set -> MkOptDescr get set a -> OptionField a

Create an option taking a single OptDescr. No explicit Name is given for the Option, the name is the first LFlag given.



:: Name 
-> get 
-> set 
-> [get -> set -> OptDescr a]

MkOptDescr constructors partially applied to flags and description.

-> OptionField a 

Create an option taking several OptDescrs. You will have to give the flags and description individually to the OptDescr constructor.

Liftings & Projections

liftOption :: (b -> a) -> (a -> b -> b) -> OptionField a -> OptionField b

viewAsFieldDescr :: OptionField a -> FieldDescr a

to view as a FieldDescr, we sort the list of interfaces (Req > Bool > Choice > Opt) and consider only the first one.

Option Descriptions

data OptDescr a

An OptionField takes one or more OptDescrs, describing the command line interface for the field.


ReqArg Description OptFlags ArgPlaceHolder (ReadE (a -> a)) (a -> [String]) 
OptArg Description OptFlags ArgPlaceHolder (ReadE (a -> a)) (a -> a) (a -> [Maybe String]) 
ChoiceOpt [(Description, OptFlags, a -> a, a -> Bool)] 
BoolOpt Description OptFlags OptFlags (Bool -> a -> a) (a -> Maybe Bool) 

type Description = String

type SFlags = [Char]

Short command line option strings

type LFlags = [String]

Long command line option strings

type ArgPlaceHolder = String

OptDescr smart constructors

type MkOptDescr get set a = SFlags -> LFlags -> Description -> get -> set -> OptDescr a

reqArg :: Monoid b => ArgPlaceHolder -> ReadE b -> (b -> [String]) -> MkOptDescr (a -> b) (b -> a -> a) a

Create a string-valued command line interface.

reqArg' :: Monoid b => ArgPlaceHolder -> (String -> b) -> (b -> [String]) -> MkOptDescr (a -> b) (b -> a -> a) a

(String -> a) variant of "reqArg"

optArg :: Monoid b => ArgPlaceHolder -> ReadE b -> b -> (b -> [Maybe String]) -> MkOptDescr (a -> b) (b -> a -> a) a

Create a string-valued command line interface with a default value.

optArg' :: Monoid b => ArgPlaceHolder -> (Maybe String -> b) -> (b -> [Maybe String]) -> MkOptDescr (a -> b) (b -> a -> a) a

(String -> a) variant of "optArg"

noArg :: Eq b => b -> MkOptDescr (a -> b) (b -> a -> a) a

boolOpt :: (b -> Maybe Bool) -> (Bool -> b) -> SFlags -> SFlags -> MkOptDescr (a -> b) (b -> a -> a) a

boolOpt' :: (b -> Maybe Bool) -> (Bool -> b) -> OptFlags -> OptFlags -> MkOptDescr (a -> b) (b -> a -> a) a

choiceOpt :: Eq b => [(b, OptFlags, Description)] -> MkOptDescr (a -> b) (b -> a -> a) a

create a Choice option

choiceOptFromEnum :: (Bounded b, Enum b, Show b, Eq b) => MkOptDescr (a -> b) (b -> a -> a) a

create a Choice option out of an enumeration type. As long flags, the Show output is used. As short flags, the first character which does not conflict with a previous one is used.