[Haskell-cafe] 'let' keyword optional in do notation?

[Martijn Schrage]

On 08-08-12 17:27, Ertugrul Söylemez wrote:
> Vo Minh Thu <noteed at 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 at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20120808/f797c369/attachment.htm>