Haskell/Indentation

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Haskell relies on indentation to reduce the verbosity of your code, but working with the indentation rules can be a bit confusing. The rules may seem many and arbitrary, but the reality of things is that there are only one or two layout rules, and all the seeming complexity and arbitrariness comes from how these rules interact with your code. So to take the frustration out of indentation and layout, the simplest solution is to get a grip on these rules.

[edit] The golden rule of indentation

Whilst the rest of this chapter will discuss in detail Haskell's indentation system, you will do fairly well if you just remember a single rule:

Code which is part of some expression should be indented further in than the line containing the beginning of that expression

What does that mean? The easiest example is a let binding group. The equations binding the variables are part of the let expression, and so should be indented further in than the beginning of the binding group: the let keyword. So,

let
 x = a
 y = b

Although you actually only need to indent by one extra space, it's more normal to place the first line alongside the 'let' and indent the rest to line up:

let x = a
    y = b

Here are some more examples:

do foo
   bar
   baz

where x = a
      y = b

case x of
  p  -> foo
  p' -> baz

Note that with 'case' it's less common to place the next expression on the same line as the beginning of the expression, as with 'do' and 'where'. Also note we lined up the arrows here: this is purely aesthetic and isn't counted as different layout; only indentation, whitespace beginning on the far-left edge, makes a difference to layout.

Things get more complicated when the beginning of the expression doesn't start at the left-hand edge. In this case, it's safe to just indent further than the line containing the expression's beginning. So,

myFunction firstArgument secondArgument = do -- the 'do' doesn't start at the left-hand edge
  foo                                        -- so indent these commands more than the beginning of the line containing the 'do'.
  bar
  baz

Here are some alternative layouts to the above which would have also worked:

myFunction firstArgument secondArgument = 
  do foo
     bar
     baz

myFunction firstArgument secondArgument = do foo
                                             bar
                                             baz

[edit] A mechanical translation

Did you know that layout (whitespace) is optional? It is entirely possible to treat Haskell as a one-dimensional language like C, using semicolons to separate things, and curly braces to group them back.

To understand layout, you need to understand two things: where we need semicolons/braces, and how to get there from layout. The entire layout process can be summed up in three translation rules (plus a fourth one that doesn't come up very often):

1. If you see one of the layout keywords, (let, where, of, do), insert an open curly brace (right before the stuff that follows it)
2. If you see something indented to the SAME level, insert a semicolon
3. If you see something indented LESS, insert a closing curly brace
4. If you see something unexpected in a list, like where, insert a closing brace before instead of a semicolon.

to be completed: work through an example

  of a
     b
      c
     d
  where
  a
  b
  c
  do
 you
  like
 the
way
 i let myself
        abuse
       these
 layout rules

[edit] Layout in action

 do first thing
 second thing
 third thing

 do first thing
    second thing 
    third thing

[edit] do within if

What happens if we put a do expression with an if? Well, as we stated above, the keywords if then else, and everything besides the 4 layout keywords do not affect layout. So things remain exactly the same:

 if foo
    then do first thing
         second thing
         third thing
    else do something else

 if foo
    then do first thing
            second thing
            third thing
    else do something else

[edit] Indent to the first

Remember from the First Rule of Layout Translation (above) that although the keyword do tells Haskell to insert a curly brace, where the curly braces goes depends not on the do, but the thing that immediately follows it. For example, this weird block of code is totally acceptable:

         do
first thing
second thing
third thing

As a result, you could also write combined if/do combination like this:

 if foo
    then do first thing
         second thing
         third thing
    else do something else

 if foo
    then do 
     first thing
     second thing
     third thing
    else do something else

This is also the reason why you can write things like this

main = do
 first thing
 second thing

instead of

main = 
 do first thing
    second thing

Both are acceptable

[edit] if within do

This is a combination which trips up many Haskell programmers. Why does the following block of code not work?

-- why is this bad?
do first thing
   if condition
   then foo
   else bar
   third thing

Just to reiterate, the if then else block is not at fault for this problem. Instead, the issue is that the do block notices that the then part is indented to the same column as the if part, so it is not very happy, because from its point of view, it just found a new statement of the block. It is as if you had written the unsugared version on the right:

-- why is this bad?
do first thing
   if condition
   then foo
   else bar
   third thing

-- still bad, just explicitly so
do { first thing
   ; if condition
   ; then foo
   ; else bar
   ; third thing }

Naturally, the Haskell compiler is confused because it thinks that you never finished writing your if expression, before writing a new statement. The compiler sees that you have written something like if condition;, which is clearly bad, because it is unfinished. So, in order to fix this, we need to indent the bottom parts of this if block a little bit inwards

-- whew, fixed it!
do first thing
   if condition
    then foo
    else bar
   third thing

-- the fixed version without sugar
do { first thing
   ; if condition
      then foo
      else bar
   ; third thing }

This little bit of indentation prevents the do block from misinterpreting your then as a brand new expression.

[edit] References

• The Haskell Report (lexemes) - see 2.7 on layout