System.Terminal.Utility

This module provides various utilities and short-cuts.

Setting colours

set_colour :: Colour -> IO ()

Set terminal foreground colour (background is set to black).

set_colours_default :: IO ()

Set default terminal colours (DWhite on DBlack).

Printing output

putStrLnC :: Colour -> String -> IO ()

Set terminal [foreground] colour and then putStrLn a string.

putPairLn :: (Colour, String) -> (Colour, String) -> IO ()

Write a pair of text strings on a single line, with different [foreground] text colours.

highlight :: ((Colour, Colour), (Colour, Colour), (Colour, Colour)) -> Int -> String -> IO ()

Print a single line of text, with a given character highlighted in colour. Useful for, say, highlighting the location of a syntax error in an expression.

The tuple consists of three colour pairs. Each pair is a foreground/background pair. The first pair applies to the next before the nominated position, the second pair applies to the nominated position itself, and the third pair applies to any text after the nominated position.

The nominated position is given by the Int argument, with 0 being the very first character of the string. Note that if the position is off the end of the string, a blank space will be added to the end of the string and that will be highlighted.

Note that no newline is written. If you want one, you must output it yourself.

highlightLN :: ((Colour, Colour), (Colour, Colour), (Colour, Colour)) -> Int -> String -> IO ()

A version of highlight that outputs a newline after the final character of text.

Handling exceptions and errors

default_exception_handler :: Exception e => e -> IO x

A default top-level exception handler, for exceptions that fail to be caught before reaching the top level.

In a properly designed application, exceptions should be anticipated, caught and handled in the correct place. (E.g., if you try to open a file, you should anticipate the possibility of an I/O exception and catch/process this appropriately.) Thus an exception reaching the top-level of the program would indicate a programming bug, and the generated error message reflects this. On a crash, the text

 An internal program malfunction has occurred.
 Please report this as a bug to the program developers.

will be emitted on stderr, coloured bright yellow on a bright red background. The exception is then re-thrown (presumably halting the program).

with_default_exception_handler :: IO x -> IO x

Take an IO action, and run it with the default_exception_handler installed. Typically you would do something like

 main = with_default_exception_handler main2

 main2 = do ...

Now all unhandled exceptions in your program will cause a suitable message to be written to stderr.