Re: [Haskell-cafe] How to use "bracket" properly ?

On Mon, Oct 19, 2009 at 6:10 AM, Roel van Dijk wrote:

On Mon, Oct 19, 2009 at 1:44 PM, zaxis wrote:

...

oh! thanks!  But why ?

A let can introduce multiple declarations.

So this

foo = do  let x = 3  let y = 4  return $ x+ y

can also be written like

foo = do  let x = 3      y = 4 -- no let  return $ x + y

To be clear, the reason this breaks is because this is a valid let syntax: let x = 1 ; y = 2 in x + y See the semicolon? So when you put a let in a block, and it sees the semicolon at the end of the line, it is expecting another let binding. If there's a newline, then the layout rule applies and the next line is considered the start of a new layout block, even though it's at the same level. So, basically, everything gets borked up.

With explicit blocks:

foo = do {  let {x = 3; y = 4;};  return $ x + y; } _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe