Call to arms: lambda-case is stuck and needs your help

[Wolfgang Jeltsch]

Am Samstag, den 07.07.2012, 00:08 -0400 schrieb Tyson Whitehead:
> I've thought some more about this and it seems to me that there are
> two ways people might intuitively think about doing grouping via
> indentation.
> 
> 1 - the first item is on the same line and subsequent ones are lined
>     up with it
> 
>   do stmt1
>      stmt2
> 
> 2 - the first item is on a new line and subsequent ones are lined up
>     with it.
> 
>   do
>     stmt1
>     stmt2
> 
> The current layout engine is targeted at (1).  It appears to do (2),
> but it is not really reliable as things start to go south if the first
> line happened to open more than one grouping (precisely the problem
> that make '\' a group token would introduce in codes).  For an
> example, consider
>
>   let greet name = do
>     putStr "hello "
>     putStrLn name
>   in f "world"
> 
> It currently translates into
> 
>   let { greet name = do {} } putStr "hello " putStrLn name in f "world"
> 
> This results in an unituituve "Empty 'do' construct" error message.

The problem is that your example is not consistently using (2). A pure
(2)-example would look like this:

    let
        greet name = do
            putStr "hello "
            putStrLn name
    in greet "world"

And this works. :-) 

Best wishes,
Wolfgang