Re: type (++) = (<>)

"plus" symbol is natural denoting joining two pieces in almost all languages. That is why I would like not to get rid of it, but to be a bit more general. So in new code that will use the new prelude to be able to use it on broader range of containers (monoids/semigroups). I really mean: (++) :: Semigroup s => s -> s -> s About the fixity, it could retain it's original fixity. 2017-07-04 3:24 GMT+03:00 Edward Kmett :

Note: I realize nobody is directly saying that we should use (++) instead of (<>) in this conversation just yet, but I want to clear a few things up.

One of the early options when the operator (<>) was coined was to try to say we should just generalize the type of (++) instead to make it mappend. (Note: it originally was mplus, in a Haskell version long long ago, so it keeps getting repurposed!) Unfortunately, this plan ran afoul of the fact that the primary libraries using the (<>) notation at the time (pretty printing libraries) also mixed it heavily with (++), exploiting the different fixities involved. (Finding a decent fixity for (<>) was a huge chunk of the conversation at the time.)

There is a deliberate fixity difference between (++) and (<>), a good chunk of code out there mixes them that deals with pretty printing that would break pretty horribly if we just outright removed (++), and trying to do a visual search and replace for (++) with (<>) in light of them having different fixities is a very error prone process, so we aren't currently planning on deprecating the existing (++) operator any time soon. At least, nobody has put a proposal to the core libraries committee to that end.

Since the call was made to make (<>) become the new operator, we ultimately decided to leave (++) untouched, even though it could be generalized to match (<>), for much the same reason that map still exists, despite there being a more general fmap: Ultimately, there isn't a reasonable migration plan to make (++) or map become the way you define the instance involved, and at least this way the name duplication can be leveraged by the folks who want a less polymorphic combinator.

Would the world be a little tidier without map or (++) hanging about? Sure. But the hate mail levels in my inbox would skyrocket commensurately. ;)

-Edward

On Mon, Jul 3, 2017 at 5:01 PM, Erik de Castro Lopo wrote:

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Vassil Ognyanov Keremidchiev wrote:

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What do you think of making (++) the same as (<>) so we could use ++ as concatenation of any monoid, not just lists in Haskell 2020? This will be more intuitive for beginners, too.

Two symbolic operators that are synonymous seems a bit of a waste. I would much rather see (++) be deprecated in favour of (<>). At work we have a custom prelude which already does this.

Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/ _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime

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