[Haskell-cafe] Parse error

[Hans Aberg]

On 17 Jan 2010, at 11:44, Andrew Coppin wrote:

>>> Urg, but that's *ugly*. Is there no way I can reduce the amount of
>>> indentation to something more reasonable?
>>
>> main = do
>>  putStrLn "Line 1"
>>  putStrLn "Line 2"
>>
>>  let xs = do
>>        x <- [1..10]
>>        y <- [1..10]
>>        return (x+y)
>>
>>  print xs
>>
>> That better?
>
> It's an improvement. It's still not pretty, but I guess that's as  
> good as it's going to get...
>
> Maybe this is an instance of Haskell trying to tell me "if you need  
> to write a 20-line do-block in the middle of your function, you're  
> doing it wrong".

Haskell starts the new indentation level where the following lexeme is  
(Haskell-98 Report, sec. 2.7). So to reduce indentation, one must  
start a new line (already mentioned in this thread). This works in Hugs:
main =
   do
   putStrLn "Line 1"
   putStrLn "Line 2"

   let
     xs =
       do
       x <- [1..10]
       y <- [1..10]
       return (x+y)

   print xs

The "xs" on a new line looks a bit unusual, and it takes a bit more  
vertical space, but one gets nice indentation levels.

   Hans