Shellac-0.9.9: A framework for creating shell envinronments

Safe HaskellNone



This module defines the Shellac interface for shell backends. A shell backend is required to provide sensible implementations for outputString, flushOutput, getSingleChar, getInput, and getWordBreakChars. All other operations may be noops (however, they must not denote bottom!).

This module is intended for use by backend implementers. It is not intended to be used by regular clients of the library. The Shellac package provides a basic backend (System.Console.Shell.Backend.Basic). More advanced backends are available in separate packages.



type CompletionFunction = (String, String, String) -> IO (Maybe (String, [String])) Source #

The type of completion functions. The argument is a triple consisting of (before,word,after), where 'word' is a string of non-word-break characters which contains the cursor position. 'before' is all characters on the line before 'word' and 'after' is all characters on the line after word. The return value should be 'Nothing' if no completions can be generated, or 'Just (newWord,completions)' if completions can be generated. 'newWord' is a new string to replace 'word' on the command line and 'completions' is a list of all possible completions of 'word'. To achieve the standard "complete-as-far-as-possible" behavior, 'newWord' should be the longest common prefix of all words in 'completions'.

data BackendOutput Source #

A datatype representing ouput to be printed. The different categories of output are distinguished to that shell backends can, for example, apply different colors or send output to different places (stderr versus stdout).


RegularOutput String

The most regular way to produce output

InfoOutput String

An informative output string, such as command help

ErrorOutput String

An string generated by an error

data ShellBackend bst Source #

This record type contains all the functions that Shellac allows the pluggable backend to provide. Most of these operations are optional and relate to advanced features like command completion and history. However, a shell backend is required to provide sensible implementations for outputString, flushOutput, getSingleChar, getInput, and getWordBreakChars.




  • initBackend :: IO bst

    Provides the backend a way to perform any necessary initialization before the shell starts. This function is called once for each shell instance. The generated value will be passed back in to each call of the other methods in this record.

  • shutdownBackend :: bst -> IO ()

    Called when the shell exits to allow the backend to perform any necessary cleanup actions.

  • outputString :: bst -> BackendOutput -> IO ()

    Causes the string to be sent to the underlying console device.

  • flushOutput :: bst -> IO ()

    Perform any operations necessary to clear any output buffers. After this operation, the user should be able to view any output sent to this backend.

  • getSingleChar :: bst -> String -> IO (Maybe Char)

    Retrieve a single character from the user without waiting for carriage return.

  • getInput :: bst -> String -> IO (Maybe String)

    Print the prompt and retrieve a line of input from the user.

  • addHistory :: bst -> String -> IO ()

    Add a string to the history list.

  • setWordBreakChars :: bst -> String -> IO ()

    Set the characters which define word boundaries. This is mostly used for defining where completions occur.

  • getWordBreakChars :: bst -> IO String

    Get the current set of word break characters.

  • onCancel :: bst -> IO ()

    A callback to run whenever evaluation or a command is canceled by the keyboard signal

  • setAttemptedCompletionFunction :: bst -> CompletionFunction -> IO ()

    A completion function that is tried first.

  • setDefaultCompletionFunction :: bst -> Maybe (String -> IO [String]) -> IO ()

    An alternate function to generate completions. The function given takes the word as an argument and generates all possible completions. This function is called (if set) after the attemptedCompletionFunction if it returns 'Nothing'.

  • completeFilename :: bst -> String -> IO [String]

    A backend-provided method to complete filenames.

  • completeUsername :: bst -> String -> IO [String]

    A backend-provided method to complete usernames.

  • clearHistoryState :: bst -> IO ()

    An operation to clear the history buffer.

  • setMaxHistoryEntries :: bst -> Int -> IO ()

    Sets the maximum number of entries managed by the history buffer.

  • getMaxHistoryEntries :: bst -> IO Int

    Gets the maximum number of entries managed by the history buffer.

  • readHistory :: bst -> FilePath -> IO ()

    Read the history buffer from a file. The file should be formatted as plain-text, with each line in the file representing a single command entered, most recent commands at the bottom. (This format is what readline produces)

  • writeHistory :: bst -> FilePath -> IO ()

    Write the history buffer to a file. The file should be formatted in the same way as in the description for readHistory.

defaultWordBreakChars :: [Char] Source #

Provides a sane default set of characters to use when breaking lines into 'words'. If a backend does not have configurable word break characters, then getWordBreakCharacters can just return this default set.

templateBackend :: a -> ShellBackend a Source #

This backend template is useful for defining custom backends. The idea is that you will use templateBackend to generate a bare-bones backend implemenation and only fill in the methods that you wish to define using the record update syntax. The parameter to templateBackend becomes the backend state associated with the backend and is passed into to each of the operation methods.