Re: [Haskell-cafe] Reinventing the wheel? Does any existing package provide an applicatively lifted (>>) ?

It should be free instead of tree: http://pointfree.io/ Regards, On Thu, Sep 28, 2017 at 4:27 PM, Jeffrey Brown wrote:

...

or, equivalently:

a `andthen` b = (>>) <$> a <*> b

for which http://pointree.io dutifully gives me:

andthen = (<*>) . ((>>) <$>)

That link is not working for me, and Google isn't finding it, and it sounds like a useful thing.

On Thu, Sep 28, 2017 at 1:55 AM, Viktor Dukhovni wrote:

...

When generating a report file from a database I found it much more efficient (significantly shorter runtime) to represent each row by an I/O action that prints the row, rather than to construct a Row object that to print and throw away.

But the naive way to construct the I/O action can be tedious to maintain once the column count gets appreciably high:

newtype Foo = Foo { _foo :: IO () } instance FromRow Foo where fromRow = Foo <$> (rowPrinter <$> field <*> field <*> field <*> ... <*> field) where rowPrinter :: Type1 -> Type2 -> Type3 -> ... -> TypeN -> IO () rowPrinter p1 p2 p3 ... pN = do printP1 printP2 printP3 ... printPN

So I decided to applicatively decompose the rowPrinter function (with the actual name of "andthen" to be determined later) as:

rowPrinter = (printP1 <$> field) `andthen` (printP2 <$> field) `andthen` (printP3 <$> field) `andthen` ... (printPN <$> field)

which avoids the need to package the column printers explicitly into a single function, and may be somewhat more efficient a well.

What was not immediately obvious to me was whether there's an "off the shelf" implementation of "andthen" I could just reuse. The necessary operator satisfies:

andthen (f (m a)) (f (m b)) = f (ma >> mb)

or, equivalently:

a `andthen` b = (>>) <$> a <*> b

for which http://pointree.io dutifully gives me:

andthen = (<*>) . ((>>) <$>)

Its type signature is:

Prelude> :set prompt "l> " l> :m + Control.Applicative l> :m + Control.Monad l> :t ((<*>) . ((>>) <$>)) ((<*>) . ((>>) <$>)) :: (Monad m, Applicative f) => f (m a) -> f (m b) -> f (m b) l>

It seems to me that this would have been done before, and the operator would already be present in some package, but I'm having trouble finding it.

(Due to the hidden constructors of FromRow defining a Semigroup does not work out here, so I can't use (<>), which also inconveniently conflicts with Monoid (<>)).

So my question is whether the operator in question is already available, under some name in some package, or else suggested names for it if new.

-- Viktor.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Jeff Brown | Jeffrey Benjamin Brown Website https://msu.edu/~brown202/ | Facebook https://www.facebook.com/mejeff.younotjeff | LinkedIn https://www.linkedin.com/in/jeffreybenjaminbrown(spammy, so I often miss messages here) | Github https://github.com/jeffreybenjaminbrown

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Sibi, sibi@psibi.in WWW: psibi.in http://www.psibi.in Twitter/github/identi.ca: psibi GPG Fingerpint: A241 B3D6 F4FD D40D D7DE B1C6 D19E 3E0E BB55 7613 Registered Linux User ID: 534664