Desugaring do-notation to Applicative

[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, <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****
>
> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users at haskell.org
> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/glasgow-haskell-users/attachments/20131002/3223ce2c/attachment.html>