Re: Desugaring do-notation to Applicative

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 :

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, 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@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

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users