Update on GHC 6.12.1

[kahl at cas.mcmaster.ca]

Simon Peyton Jones wrote:
 > 
 > Recursive do-notation.
 > ~~~~~~~~~~~~~~~~~~~~~~
 > The change is this.  Instead of writing
 > 
 >   mdo { a <- getChar
 >       ; b <- f c
 >       ; c <- g b
 >       ; putChar c
 >       ; return b }
 > 
 > you would write
 > 
 >   do { a <- getChar
 >      ; rec { b <- f c
 >            ; c <- g b }
 >      ; putChar c
 >      ; return b }
 > 
 > That is, 
 >   * 'mdo' is eliminated 
 >   * 'rec' is added, which groups a bunch of statements
 >     into a single recursive statement
 > See http://hackage.haskell.org/trac/ghc/ticket/2798
 > 
 > This 'rec' thing is already present for the arrow notation, so it  
 > makes the two more uniform.  Moreover, 'rec' lets you say more
 > precisely where the recursion is (if you want to), whereas 'mdo' just
 > says "there's recursion here somewhere".  Lastly, all this works with
 > rebindable syntax (which mdo does not).
 > 
 > The main question is not whether to make this change, but when. 

This last point notwithstanding,
I find the scoping rules very unintuitive!
(b and c appear to escape their apparently nested scope.)


Wolfram