Difference between revisions of "Emacs/Indentation"
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| − | {{Haskell infobox}} |
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| − | = Indentation Approaches = |
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| − | Emacs can indent Haskell in various ways. The most common is the tab cycle. |
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| − | == Indentation using tab cycle == |
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| − | Haskell-mode offers intelligent indentation. As Haskell source code uses indentation aware code blocks, there is usually more than one column for which indentation makes sense. |
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| − | Hit tab a few times to see a few different indentation possibilities. |
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| − | For example, imagine the following is open in a haskell-mode buffer, where <code>!</code> represents the point: |
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| − | <haskell> |
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| − | foo :: Int -> String |
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| − | foo 0 = f 4 ++ s |
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| − | where f 4 = "hello" ++ |
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| − | ! |
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| − | </haskell> |
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| − | If you ask <code>haskell-mode</code> to indent for you, where should it indent to? There are four basic options: |
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| − | <ol> |
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| − | <li> |
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| − | You want to finish off the expression you were writing in the last line. <code>Haskell-mode</code> indents to be underneath the <code>"</code> character at the beginning of <code>"hello"</code>: |
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| − | <haskell> |
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| − | where f 4 = "hello" ++ |
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| − | ! |
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| − | </haskell> |
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| − | This is debatably a bad choice as you'd probably want to indent a bit further in to make it clear that you were carrying on an expression, but the layout rule would accept something like the following: |
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| − | <haskell> |
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| − | where f 4 = "hello" ++ |
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| − | "world" |
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| − | </haskell> |
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| − | </li> |
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| − | <li> |
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| − | You want to add a second equation for <code>f</code>. <code>Haskell-mode</code> will indent to line up with the first argument, and fill in the <code>f</code> in the equation: |
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| − | <haskell> |
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| − | where f 4 = "hello" ++ |
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| − | f ! |
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| − | </haskell> |
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| − | This is an unlikely choice as the expression in the previous line isn't complete, but <code>haskell-mode</code> isn't smart enough to know that. (If <code>f</code> had been something without arguments, like <hask>where f = "hello"</hask>, then it's impossible to have more than one equation and haskell-mode won't offer this indentation level.) |
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| − | </li> |
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| − | <li> |
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| − | You want to add a second binding to the <code>where</code>-block. Haskell-mode indents to line up with the <code>f</code>: |
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| − | <haskell> |
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| − | where f 4 = "hello" ++ |
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| − | ! |
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| − | </haskell> |
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| − | |||
| − | </li> |
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| − | <li>You want to start an entirely new top-level binding. Haskell-mode indents to the first column: |
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| − | <haskell> |
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| − | foo :: Int -> String |
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| − | foo 0 = f 4 ++ s |
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| − | where f 4 = "hello" ++ |
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| − | ! |
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| − | </haskell> |
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| − | |||
| − | </li> |
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| − | </ol> |
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| − | These four locations can be reached by repeatedly pressing <code>TAB</code>. This is what's known as the tab-cycle. The innermost location is offered first, then cycling progresses outwards. Although this may seem like an inefficient system (and it is indeed a shame that Haskell's design didn't result in an unambiguous indentation system), you do quickly get used to the tab-cycle and indenting Haskell code. |
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| − | ''Notes:'' |
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| − | Do not use indent-region |
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| − | Using indent-region is generally a bad idea on Haskell code, because it would need to know which of the tab-cycle stops you wish to choose for each line. The innermost one is chosen in each case, which often results in unusable code. Moral: just don't use indent-region with haskell-mode. |
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| − | == Using rectangular region commands == |
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| − | Emacs has a set of commands which operate on the region as if it were rectangular. This turns out to be extremely useful when dealing with whitespace sensitive languages. |
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| − | <code>C-x r o</code> is "Open Rectangle". It will shift any text within the rectangle to the right side. Also see: |
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| − | <code>C-x r t</code> is "String Rectangle". It will shift any text within the rectangle over to the right, and insert a given string prefixing all the lines in the region. If comment-region didn't already exist, you could use this instead, for example. |
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| − | <code>C-x r d</code> is "Delete Rectangle". It will delete the contents of the rectangle and move anything on the right over. |
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| − | <code>C-x r r</code> is "Copy Rectangle to Register". It will prompt you for a register number so it can save it for later. |
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| − | <code>C-x r g</code> is "Insert register". This will insert the contents of the given register, overwriting whatever happens to be within the target rectangle. (So make room) |
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| − | <code>C-x r k</code> is "Kill rectangle". Delete rectangle and save contents for: |
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| − | <code>C-x r y</code> is "Yank rectangle". This will insert the contents of |
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| − | the last killed rectangle. |
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| − | As with all Emacs modifier combos, you can type <code>C-x r C-h</code> to find out what keys are bound beginning with the <code>C-x r</code> prefix. |
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| − | == Aligning code == |
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| − | Emacs22 has a neat tool called: align-regexp. Select a region you want to align text within, M-x align-regexp, and type a regexp representing the alignment delimiter. |
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| − | For example, I often line up my Haddock comments: |
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| − | <haskell> |
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| − | f :: a -- ^ does a |
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| − | -> Foo b -- ^ and b |
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| − | -> c -- ^ to c |
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| − | </haskell> |
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| − | Select the region, and let the regexp be: <code>--</code> |
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| − | <haskell> |
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| − | f :: a -- ^ does a |
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| − | -> Foo b -- ^ and b |
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| − | -> c -- ^ to c |
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| − | </haskell> |
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| − | Of course, this works for just about anything. Personally, I've globally bound it to <code>C-x a r</code>: |
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| − | <code>(global-set-key (kbd "C-x a r") 'align-regexp)</code>. |
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| − | Note that you also just use these Haskell rules for use with the aligner: |
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| − | <code> |
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| − | (add-to-list 'align-rules-list |
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| − | '(haskell-types |
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| − | (regexp . "\\(\\s-+\\)\\(::\\|∷\\)\\s-+") |
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| − | (modes quote (haskell-mode literate-haskell-mode)))) |
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| − | (add-to-list 'align-rules-list |
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| − | '(haskell-assignment |
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| − | (regexp . "\\(\\s-+\\)=\\s-+") |
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| − | (modes quote (haskell-mode literate-haskell-mode)))) |
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| − | (add-to-list 'align-rules-list |
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| − | '(haskell-arrows |
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| − | (regexp . "\\(\\s-+\\)\\(->\\|→\\)\\s-+") |
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| − | (modes quote (haskell-mode literate-haskell-mode)))) |
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| − | (add-to-list 'align-rules-list |
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| − | '(haskell-left-arrows |
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| − | (regexp . "\\(\\s-+\\)\\(<-\\|←\\)\\s-+") |
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| − | (modes quote (haskell-mode literate-haskell-mode)))) |
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| − | </code> |
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