Re: data-default WAS: [containers] Proposal: Change to the Data.Map Monoid

I don't find Default for things like ParserOptions that compelling at all. IMHO Henning's approach is the best overall. In practice the problem I have with deriving the instance generically is that it just doesn't work. I commonly have several Bool values, and possibly some Ints or Doubles, and they all need different defaults. The only way to make that work would be newtypeing over Bool and writing the default instances by hand. No thanks, just writing a straightforward value is much nicer. John L. On May 22, 2014 6:00 AM, "Roman Cheplyaka" wrote:

I don't agree to Empty either. As I mentioned, the most compelling use

Default I see are things like ParserOptions and ConnectionInfo.

Empty could be a separate typeclass with a different set of instances. It should probably come with laws (its monofoldable length/size should be 0).

Roman

* João Cristóvão [2014-05-22 12:36:24+0100]

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Hi Roman, Daniel,

I do see your point, and I don't find it a problem. In those particular cases perhaps it only makes sense to define a Default value for the same cases as mempty applies, namely for Sum and Product.

So, I'm fine with not including default values for Int, Bool, Float...

However, I'm not sure I agree with calling it Empty. You might want to use Default with your own Algebraic data type where

cases for the

...

concept of Default makes sense, but the concept of Empty does not. The Empty name may be a little too misleading (as mempty is already for Monoid, see the Product newtype).

Cheers, João

2014-05-22 9:10 GMT+01:00 Daniel Trstenjak :

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Hi João,

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... with an obvious default value ...

I think that's the problem some people have, that the default values for several types aren't obvious at all and therefore you have to know the default values of several types to easily read code using 'def'.

The default values for container like types - like String or List -

are

...

mostly quite obvious, they're just the empty ones. But the default values for types like Int, Bool of Float aren't that obvious and they certainly don't make sense in any use case.

That's also the reason why an Empty type class was advocated instead of the Default one, because than it's really obvious what the result for container like types is.

Greetings, Daniel _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

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