Alternative Proxy

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Fumiaki Kinoshita

9 Mar 2016 9 Mar '16

5:49 p.m.

This is the only possible definition...so it should exist instance Alternative Proxy where empty = Proxy _ <|> _ Proxy

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David Feuer

9 Mar 9 Mar

6:12 p.m.

+1. I see no problem here. On Wed, Mar 9, 2016 at 5:49 PM, Fumiaki Kinoshita wrote:

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This is the only possible definition...so it should exist

instance Alternative Proxy where empty = Proxy _ <|> _ Proxy

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amindfv＠gmail.com

7:48 p.m.

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El 9 mar 2016, a las 17:49, Fumiaki Kinoshita escribió:

This is the only possible definition...so it should exist

Although I have nothing particularly against an Alternative instance for Proxy, I'd like to say I think "this is the only possible definition, so it should exist" is a nonsequitur. If (again, not specifically speaking about "instance Alternative Proxy" here) there is an instance method with no likely practical use, the fact that the compiler yells when you use it is a *feature*. Tom

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instance Alternative Proxy where empty = Proxy _ <|> _ Proxy _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Fumiaki Kinoshita

8:28 p.m.

Thinking an instance non-practical does not imply we *never* use the instance. The lack of a uniquely legal instance turns out to be a problem at some point, maybe right now.

amindfv＠gmail.com

9:12 p.m.

My point is that there's a cost to adding instances -- namely that accidental calling of the class functions (e.g. after refactoring) isn't caught by the compiler. Since there's a cost, we need to make sure there's a larger benefit before we add them -- e.g. a hypothetical or real use-case. Tom

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El 9 mar 2016, a las 20:28, Fumiaki Kinoshita escribió:

Thinking an instance non-practical does not imply we *never* use the instance. The lack of a uniquely legal instance turns out to be a problem at some point, maybe right now.

Fumiaki Kinoshita

9:28 p.m.

There's a cost to __not__ adding instances as well, namely that accidental need of the class functions (e.g. after abstraction) doesn't fit the library. Since there's a cost, we need to make sure there's a larger benefit before we decide not to add them -- e.g. a hypothetical or real risk. 2016-03-09 18:12 GMT-08:00 :

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My point is that there's a cost to adding instances -- namely that accidental calling of the class functions (e.g. after refactoring) isn't caught by the compiler.

Since there's a cost, we need to make sure there's a larger benefit before we add them -- e.g. a hypothetical or real use-case.

Tom

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El 9 mar 2016, a las 20:28, Fumiaki Kinoshita escribió:

Thinking an instance non-practical does not imply we *never* use the instance. The lack of a uniquely legal instance turns out to be a problem at some point, maybe right now.

Edward Kmett

10:13 p.m.

We actually already put in a patch for this about 10 days ago, when the discussion came up around U1: https://ghc.haskell.org/trac/ghc/ticket/11650 -Edward On Wed, Mar 9, 2016 at 5:49 PM, Fumiaki Kinoshita wrote:

...

This is the only possible definition...so it should exist

instance Alternative Proxy where empty = Proxy _ <|> _ Proxy

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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amindfv＠gmail.com
David Feuer
Edward Kmett
Fumiaki Kinoshita