I actually do that too but find [this](https://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md) better for the following anecdotal reason:

Renaming something shouldn't require re-indentation.

If you do it this way, you can rename `HTTPException` and not have to move anything else. If you do it this way:

```haskell
data HTTPException = NotFound
                   | Whatever
                   | Whatever
```

Renaming `HTTPException` such that its length changes requires moving the following lines around too.

There's also this way:

```haskell
data HTTPException =
      NotFound
    | Whatever
    | Whatever
```

But I find that just annoying to type.