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Emacs/Indentation

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[[Category:Emacs|*]]
 
{{Haskell infobox}}
 
  
= Indentation Approaches =
 
 
Emacs can indent Haskell in various ways. The most common is the tab cycle.
 
 
== Indentation using tab cycle ==
 
 
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.
 
 
Hit tab a few times to see a few different indentation possibilities.
 
 
For example, imagine the following is open in a haskell-mode buffer, where <code>!</code> represents the point:
 
 
<haskell>
 
foo :: Int -> String
 
foo 0 = f 4 ++ s
 
  where f 4 = "hello" ++
 
!
 
</haskell>
 
 
If you ask <code>haskell-mode</code> to indent for you, where should it indent to? There are four basic options:
 
 
<ol>
 
<li>
 
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>:
 
 
<haskell>
 
where f 4 = "hello" ++
 
            !
 
</haskell>
 
 
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:
 
 
<haskell>
 
where f 4 = "hello" ++
 
            "world"
 
</haskell>
 
 
</li>
 
<li>
 
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:
 
 
<haskell>
 
where f 4 = "hello" ++
 
      f !
 
</haskell>
 
 
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.)
 
</li>
 
<li>
 
You want to add a second binding to the <code>where</code>-block. Haskell-mode indents to line up with the <code>f</code>:
 
 
<haskell>
 
where f 4 = "hello" ++
 
      !
 
</haskell>
 
 
</li>
 
<li>You want to start an entirely new top-level binding. Haskell-mode indents to the first column:
 
 
<haskell>
 
foo :: Int -> String
 
foo 0 = f 4 ++ s
 
  where f 4 = "hello" ++
 
!
 
</haskell>
 
 
</li>
 
</ol>
 
 
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.
 
 
''Notes:''
 
 
Do not use indent-region
 
 
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.
 
 
== Using rectangular region commands ==
 
 
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.
 
 
<code>C-x r o</code> is "Open Rectangle".  It will shift any text within the rectangle to the right side.  Also see:
 
 
<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.
 
 
<code>C-x r d</code> is "Delete Rectangle".  It will delete the contents of the rectangle and move anything on the right over.
 
 
<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.
 
 
<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)
 
 
<code>C-x r k</code> is "Kill rectangle".  Delete rectangle and save contents for:
 
 
<code>C-x r y</code> is "Yank rectangle".  This will insert the contents of
 
the last killed rectangle.
 
 
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.
 
 
== Aligning code ==
 
 
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.
 
 
For example, I often line up my Haddock comments:
 
 
<haskell>
 
f :: a -- ^ does a
 
  -> Foo b -- ^ and b
 
  -> c -- ^ to c
 
</haskell>
 
 
Select the region, and let the regexp be: <code>--</code>
 
 
<haskell>
 
f :: a    -- ^ does a
 
  -> Foo b -- ^ and b
 
  -> c    -- ^ to c
 
</haskell>
 
 
Of course, this works for just about anything.  Personally, I've globally bound it to <code>C-x a r</code>:
 
 
<code>(global-set-key (kbd "C-x a r") 'align-regexp)</code>.
 

Latest revision as of 11:57, 11 April 2016