Shellac- A framework for creating shell envinronments




This module implements a framework for creating read-eval-print style command shells. Shells are created by declaratively defining evaluation functions and "shell commands". Input is read using a pluggable backend. The shell framework handles command history and word completion if the backend supports it.

The basic idea is for creating a shell is:

  1. Create a list of shell commands and an evaluation function
  2. Create a shell description (using mkShellDescription)
  3. Set up the initial shell state
  4. Run the shell (using runShell)

Shell commands and the evaluation function are written in a custom monad. See System.Console.Shell.ShellMonad for details on using this monad.


Shell Descriptions

data ShellDescription st Source

A record type which describes the attributes of a shell.




shellCommands :: [ShellCommand st]

Commands for this shell

commandStyle :: CommandStyle

The style of shell commands

evaluateFunc :: String -> Sh st ()

The evaluation function for this shell

greetingText :: Maybe String

Text to print when the shell starts

wordBreakChars :: [Char]

The characters upon which the backend will break words

beforePrompt :: Sh st ()

A shell action to run before each prompt is printed

prompt :: st -> IO String

A command to generate the prompt to print

secondaryPrompt :: Maybe (st -> IO String)

A command to generate the secondary prompt. The secondary prompt is used for multi-line input. If not set, the regular prompt is used instead.

exceptionHandler :: ShellacException -> Sh st ()

A set of handlers to call when an exception occurs

defaultCompletions :: Maybe (st -> String -> IO [String])

If set, this function provides completions when NOT in the context of a shell command

historyFile :: Maybe FilePath

If set, this provides the path to a file to contain a history of entered shell commands

maxHistoryEntries :: Int

The maximum number of history entries to maintain

historyEnabled :: Bool

If true, the history mechanism of the backend (if any) will be used; false will disable history features.

initialShellDescription :: ShellDescription stSource

A basic shell description with sane initial values.

mkShellDescription :: [ShellCommand st] -> (String -> Sh st ()) -> ShellDescription stSource

Creates a simple shell description from a list of shell commands and an evaluation function.

defaultExceptionHandler :: ShellacException -> Sh st ()Source

The default shell exception handler. It simply prints the exception and returns the shell state unchanged. (However, it specificaly ignores the thread killed exception, because that is used to implement execution canceling)

Executing Shells

runShell :: ShellDescription st -> ShellBackend bst -> st -> IO stSource

Run a shell. Given a shell description, a shell backend to use and an initial state this function runs the shell until it exits, and then returns the final state.

Creating Shell Commands



:: String

the name of the command

-> ShellCommand st 

Creates a shell command which will exit the shell.



:: String

the name of the command

-> ShellCommand st 

Creates a command which will print the shell help message.



:: String

command name

-> String

help message

-> (st -> Bool)


-> (Bool -> st -> st)


-> ShellCommand st 

Creates a command to toggle a boolean value



:: CommandFunction f st 
=> String

the name of the command

-> f

the command function. See CommandFunction for restrictions on the type of this function.

-> String

the help string for this command

-> ShellCommand st 

Creates a user defined shell commmand. This relies on the typeclass machenery defined by CommandFunction.

class CommandFunction f st | f -> stSource

This class is used in the cmd function to automaticly generate the command parsers and command syntax strings for user defined commands. The type of 'f' is restricted to have a restricted set of monomorphic arguments (Int, Integer, Float, Double, String, File, Username, and Completable) and the head type must be Sh st ()

  f :: Int -> File -> Sh MyShellState ()
  g :: Double -> Sh st ()
  h :: Sh SomeShellState ()

are all legal types, whereas:

  bad1 :: a -> Sh (MyShellState a) ()
  bad2 :: [Int] -> Sh MyShellState ()
  bad3 :: Bool -> MyShellState

are not.


newtype File Source

Represents a command argument which is a filename


File String 


newtype Username Source

Represents a command argument which is a username


Username String 


newtype Completable compl Source

Represents a command argument which is an arbitrary completable item. The type argument determines the instance of Completion which is used to create completions for this command argument.


Completable String 


(CommandFunction r st, Completion compl st) => CommandFunction (Completable compl -> r) st 

class Completion compl st | compl -> st whereSource

A typeclass representing user definable completion functions.


complete :: compl -> st -> String -> IO [String]Source

Actually generates the list of possible completions, given the current shell state and a string representing the beginning of the word.

completableLabel :: compl -> StringSource

generates a label for the argument for use in the help displays.

type ShellCommand st = ShellDescription st -> (String, CommandParser st, Doc, Doc)Source

The type of a shell command. The shell description is passed in, and the tuple consists of (command name,command parser,command syntax document,help message document)


type Subshell st st' = (st -> IO st', st' -> IO st, st' -> IO (ShellDescription st'))Source

The type of subshells. The tuple consists of:

  1. A function to generate the initial subshell state from the outer shell state
  2. A function to generate the outer shell state from the final subshell state
  3. A function to generate the shell description from the initial subshell state



:: (st -> IO st')


-> ShellDescription st'

A shell description for the subshell

-> IO (Subshell st st') 

Creates a simple subshell from a state mapping function and a shell description.

Printing Help Messages

showShellHelp :: ShellDescription st -> StringSource

Prints the help message for this shell, which lists all avaliable commands with their syntax and a short informative message about each.

showCmdHelp :: ShellDescription st -> String -> StringSource

Print the help message for a particular shell command

Auxiliary Types

data CommandStyle Source

Datatype describing the style of shell commands. This determines how shell input is parsed.



Indicates that all input is to be interpreted as shell commands; input is only passed to the evaluation fuction if it cannot be parsed as a command.

CharPrefixCommands Char

Indicates that commands are prefixed with a particular character. Colon ':' is the default character (a la GHCi).


Commands consist of a single character.

data ShellSpecial st Source

Special commands for the shell framework.



Causes the shell to exit

ShellHelp (Maybe String)

Causes the shell to print an informative message. If a command name is specified, only information about that command will be displayed


Instructs the shell to do nothing; redisplay the prompt and continue

ShellContinueLine String

Ask the shell to continue accepting input on another line, which should be appended to the given string

forall st' . ExecSubshell (Subshell st st')

Causes the shell to execute a subshell

type OutputCommand = BackendOutput -> IO ()Source

The type of commands which produce output on the shell console.

type CommandResult st = (st, Maybe (ShellSpecial st))Source

The type of results from shell commands. They are a modified shell state and possibly a shell "special" action to execute.

type ShellacException = SomeExceptionSource

Compatability layer. For base-3, this is 'Exception'. For base-4, this is 'SomeException'.