Desugaring do-notation to Applicative

[Dominique Devriese]

Perhaps an alternative for this could be extending McBride's idiom brackets:
  https://personal.cis.strath.ac.uk/conor.mcbride/pub/she/idiom.html
with a form of top-level let, something like:

(| let x = expr1
        y = expr2
        z = expr3
   in x*y + y*z + z*x |)
=
pure (\x y z -> x*y + y*z + z*x) <*> expr1 <*> expr2 <*> expr3

This seems like it would nicely commute with the desugaring of "let x
= e1 in e2" into "(\x -> e2) e1":

(| let x = expr1
        y = expr2
        z = expr3
   in x*y + y*z + z*x |)
=
(| (\x y z -> x*y + y*z + z*x) expr1 expr2 expr3 |)
=
pure (\x y z -> x*y + y*z + z*x) <*> expr1 <*> expr2 <*> expr3

Regards,
Dominique

2013/10/2 Dan Doel <dan.doel at gmail.com>:
> Unfortunately, in some cases, function application is just worse. For
> instance, when the result is a complex arithmetic expression:
>
>     do x <- expr1; y <- expr2; z <- expr3; return $ x*y + y*z + z*x
>
> In cases like this, you have pretty much no choice but to name intermediate
> variables, because the alternative is incomprehensible. But applicative
> notation:
>
>     (\x y z -> x*y + y*z + z*x) <$> expr1 <*> expr2 <*> expr3
>
> moves the variable bindings away from the expressions they're bound to, and
> we require extra parentheses to delimit things, and possibly more.
>
> Desugaring the above do into applicative is comparable to use of plain let
> in scheme (monad do is let*, mdo was letrec). And sometimes, let is nice,
> even if it has an equivalent lambda form.
>
> And as Jake mentioned, syntax isn't the only reason for Applicative.
> Otherwise it'd just be some alternate names for functions involving Monad.
>
>
>
> On Wed, Oct 2, 2013 at 5:12 AM, <p.k.f.holzenspies at utwente.nl> wrote:
>>
>> I thought the whole point of Applicative (at least, reading Connor?s
>> paper) was to restore some function-application-style to the whole
>> effects-thing, i.e. it was the very point *not* to resort to binds or
>> do-notation.
>>
>>
>>
>> That being said, I?m all for something that will promote the use of the
>> name ?pure? over ?return?.
>>
>>
>>
>> +1 for the Opt-In
>>
>>
>>
>> Ph.
>>
>>
>>
>>
>>
>>
>>
>> From: Glasgow-haskell-users
>> [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of Iavor
>> Diatchki
>>
>>
>>
>> do x1 <- e1
>>
>>
>>
>>    -- The following part is `Applicative`
>>
>>    (x2,x3) <- do x2 <- e2 x1
>>
>>                  x3 <- e3
>>
>>                  pure (x2,x3)
>>
>>
>>
>>    f x1 x2 x3
>>
>>
>> _______________________________________________
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>> Glasgow-haskell-users at haskell.org
>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>>
>
>
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