---------- Forwarded message ---------- From: David Feuer <david.fe...@gmail.com> Date: Wed, Aug 8, 2012 at 12:22 PM Subject: Re: [Haskell-cafe] 'let' keyword optional in do notation? To: Martijn Schrage <mart...@oblomov.com>
Changing scoping rules based on whether things are right next to each other? No thanks. On Wed, Aug 8, 2012 at 11:44 AM, Martijn Schrage <mart...@oblomov.com> wrote: > On 08-08-12 17:27, Ertugrul Söylemez wrote: > > Vo Minh Thu <not...@gmail.com> wrote: > > This is not a parsing problem, but a scoping one: try to run this > program: > > main = do > let x = y > y = 5 > let a = b > let b = 6 > print (x, y, a, b) > > Cheers, > Thu > > Martijn has actually covered this question: > > Where each sequence of let-less bindings is put in a separate > binding group. I'm no parsing wizard, but I couldn't come up with > any situations in which this would cause ambiguity. To me, the > let-less version is easier on the eyes, more consistent with <- > bindings, and also makes it less of a hassle to move stuff around. > > > To make it more clear, this is the transformation I propose: > > do ... -- not a let-less binding > x1 = exp1 -- \ > .. -- only let-less bindings > xn = expn -- / > ... -- not a let-less binding > > becomes > > do ... > let x1 = exp1 > .. > xn = expn > ... > > So > > main = do > > x = y > y = 5 > a = b > > b = 6 > print (x, y, a, b) > > > would put everything in the same binding group and compile successfully. To > get Thu's example, you would write > > main = do > > x = y > y = 5 > let a = b > let b = 6 > print (x, y, a, b) > > The last let could even be left out. > > Cheers, Martijn > > The suggestion seems sound to me, and the additional 'let' can really be > annoying in cases where you have a lot of 'let' bindings among very few > monadic actions. My current way to deal with this is to move the stuff > to separate computations, but it's certainly not a nice solution: > > myComp = c >>= f > where > f x = ... > where > a = ... > b = ... > > > Greets > Ertugrul > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
