violation of Haskell indentation rules for if-then-else

Question

According to the Haskell indentation rules, "Code which is part of some expression should be indented further in than the beginning of that expression". However, I found the following example, which seems to violate the rule above, compiles without any error or warning:

someFunction :: Bool -> Int -> Int -> Int
someFunction condition a b = if condition
then a - b
      else a + b

Here I am defining a function someFunction, its body is an if-then-else block. According to the indentation rule, the then block is a part of the same expression in the first line, so it should be indented further than its previous line. Yet in my example, the second line then starts at the same column as the first line, and this example compiles.

I am not sure what is going on here. I am working with GHC version 8.0.1.

Answer 1

I'm reasonably sure this is an artifact of a deliberate GHC variation on the indentation rule. Nice catch!

GHC reads this

foo = do
  item
  if a
  then b
  else c
  item

as

foo = do {
  item ;
  if a ;
  then b ;
  else c ;
  item }

which should trigger a parse error.

However, this was so common that at a certain point the GHC devs decided to allow for an optional ; before then and else. This change to the if grammar makes the code compile.

This means that if became "special", in that it does not have to indented more, but only as much as the previous item. In the code posted in the question, then is indented as much as the previous item, so there's an implicit ; before it, and that makes the code compile.

I would still try to avoid this "style", though, since it's quirky.

(Personally, I wouldn't have added this special case to GHC. But it's not a big deal, anyway.)

I now noticed that the Wikibook mentions this variant as a "proposal" for a future version of Haskell. This is a bit outdated now, and has been implemented in GHC since then.