6 Indentation

For general information about indentation support in GNU Emacs, see (emacs)Indentation.

In Haskell, code indentation has semantic meaning as it defines the block structure⁴.

As GNU Emacs’ default indentation function (i.e. indent-relative) is not designed for use with Haskell’s layout rule, Haskell mode includes indentation rules adapted to Haskell. haskell-indentation-mode binds TAB to cycle through possible indentation points based on some clever heuristics. SHIFT-TAB cycles in the reverse direction, and RET moves to a new line and indents to the first possible indentation level.

6.1 haskell-indent-mode

If you want to use the obsolete haskell-indent-mode, the recommended way is to load it in haskell-mode-hook. This can be done either by using M-x customize-variable RET haskell-mode-hook which provides a convenient user interface or by adding the following line to your .emacs file:

(add-hook 'haskell-mode-hook 'haskell-indent-mode)

This will turn off haskell-indentation-mode.

6.2 Interactive Block Indentation

haskell-indentation can move whole blocks to the left or to the right. Just mark a block and then use TAB or S-TAB.

6.3 Rectangle Commands

GNU Emacs provides so-called rectangle commands which operate on rectangular areas of text, which are particularly useful for languages with a layout rule such as Haskell. See (emacs)Rectangles, to learn more about rectangle commands.

Moreover, CUA mode (see (emacs)CUA Bindings) provides enhanced rectangle support with visible rectangle highlighting. When CUA mode is active, you can initiate a rectangle selection by C-RET and extend it simply by movement commands. You don’t have to enable full CUA mode to benefit from these enhanced rectangle commands; you can activate CUA selection mode (without redefining C-x,C-c,C-v, and C-z) by calling M-x cua-selection-mode (or adding (cua-selection-mode nil) to your haskell-mode-hook).