Emacs/Indentation
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 !
represents the point:
foo :: Int -> String
foo 0 = f 4 ++ s
where f 4 = "hello" ++
!
If you ask haskell-mode
to indent for you, where should it indent to? There are four basic options:
-
You want to finish off the expression you were writing in the last line.
Haskell-mode
indents to be underneath the"
character at the beginning of"hello"
:where f 4 = "hello" ++ !
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:
where f 4 = "hello" ++ "world"
-
You want to add a second equation for
f
.Haskell-mode
will indent to line up with the first argument, and fill in thef
in the equation:where f 4 = "hello" ++ f !
This is an unlikely choice as the expression in the previous line isn't complete, but
haskell-mode
isn't smart enough to know that. (Iff
had been something without arguments, likewhere f = "hello"
, then it's impossible to have more than one equation and haskell-mode won't offer this indentation level.) -
You want to add a second binding to the
where
-block. Haskell-mode indents to line up with thef
:where f 4 = "hello" ++ !
- You want to start an entirely new top-level binding. Haskell-mode indents to the first column:
foo :: Int -> String foo 0 = f 4 ++ s where f 4 = "hello" ++ !
These four locations can be reached by repeatedly pressing TAB
. 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.