Haskell/Indentation

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Haskell relies on indentation to reduce the verbosity of your code. Despite some complexity in practice, there are really only a couple fundamental layout rules.[1]

The golden rule of indentation[edit]

Code which is part of some expression should be indented further in than the beginning of that expression (even if the expression is not the leftmost element of the line).

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. When you start the expression on a separate line, you only need to indent by one space (although more than one space is also acceptable and may be more clear).

let
 x = a
 y = b

You may also place the first clause alongside the 'let' as long as you indent the rest to line up:

wrong wrong right
 let x = a
  y = b
 let x = a
      y = b
 let x = a
     y = b

This tends to trip up a lot of beginners: All grouped expressions must be exactly aligned. On the first line, Haskell counts everything to the left of the expression as indent, even though it is not whitespace.


Here are some more examples:

do
  foo
  bar
  baz

do foo
   bar
   baz

where x = a
      y = b

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

Note that with 'case' it is less common to place the first subsidiary expression on the same line as the 'case' keyword (although it would still be valid code). Hence, the subsidiary expressions in a case expression tend to be indented only one step further than the 'case' line. Also note how we lined up the arrows here: this is purely aesthetic and is not counted as different layout; only indentation (i.e. whitespace beginning on the far-left edge) makes a difference to the interpretation of the 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 which all work:

myFunction firstArgument secondArgument = 
  do foo
     bar
     baz

myFunction firstArgument secondArgument = do foo
                                             bar
                                             baz
myFunction firstArgument secondArgument = 
  do
     foo
     bar
     baz

Explicit characters in place of indentation[edit]

Indentation is actually optional if you instead use semicolons and curly braces for grouping and separation, as in "one-dimensional" languages like C. Even though the consensus among Haskell programmers is that meaningful indentation leads to better-looking code, understanding how to convert from one style to the other can help understand the indentation rules. 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.

For instance, this definition...

foo :: Double -> Double
foo x =
    let s = sin x
        c = cos x
    in 2 * s * c

...can be rewritten without caring about the indentation rules as:

foo :: Double -> Double;
foo x = let {
  s = sin x;
  c = cos x;
  } in 2 * s * c

One circumstance in which explicit braces and semicolons can be convenient is when writing one-liners in GHCi:

Prelude> let foo :: Double -> Double; foo x = let { s = sin x; c = cos x } in 2 * s * c
Exercises

Rewrite this snippet from the Control Structures chapter using explicit braces and semicolons:

doGuessing num = do
  putStrLn "Enter your guess:"
  guess <- getLine
  case compare (read guess) num of
    LT -> do putStrLn "Too low!"
             doGuessing num
    GT -> do putStrLn "Too high!"
             doGuessing num
    EQ -> putStrLn "You Win!"

Layout in action[edit]

wrong wrong right right
 do first thing
 second thing
 third thing
 do first thing
  second thing
  third thing
 do first thing
    second thing 
    third thing
 do
   first thing
   second thing 
   third thing

Indent to the first[edit]

Due to the "golden rule of indentation" described above, a curly brace within a do block depends not on the do itself but the thing that immediately follows it. For example, this weird-looking 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:

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

It isn't about the do, it's about lining up all the items that are at the same level within the do.

Thus, all of the following are acceptable:

main = do
 first thing
 second thing

or

main = 
 do
   first thing
   second thing

or

main = 
 do first thing
    second thing

if within do[edit]

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

sweet but wrong unsweet and wrong
-- 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 bad because it is unfinished. In order to fix this, we need to indent the bottom parts of this if block so that then and else become part of the if statement.

sweet and correct unsweet and correct
-- 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 }

Now, the do block sees the whole if statement as one item. When if-then-else statements are not within do blocks, this specific indentation isn't technically necessary, but it never hurts, so it is a good habit to always indent if-then-else in this way.

Exercises
The if-within-do issue has tripped up so many Haskellers that one programmer has posted a proposal to the Haskell prime initiative to add optional semicolons between if then else. How would that help?


Issues with indentation are explained further in connection with showing how do is syntactic sugar for the monadic operater (>>=). See Translating the bind operater and the associated footnote about indentation.


Notes[edit]

  1. See section 2.7 of The Haskell Report (lexemes) on layout.