Re: [Haskell-cafe] What Haskell Records Need

I'm new to Haskell, but I do like your idea. I prefer this as a built-in feature because it will create a standard way of doing this, making the question "wich package should I use to get mutatos? lens-foo, lens-bar, monad-lens, lens-lens-foo-bar, ...?" simply go away. So, yes, I up-vote your idea to write an official proposal. Thiago. 2012/8/2 Jonathan Geddes :

Richard O'Keefe Said:

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Ouch! And that's not even very deeply nested. Imagine 4 or 5 levels deep. It really makes Haskell feel clunky next to `a.b.c.d = val` that you see in other languages.

I was taught that this kind of thing violates the Law of Demeter and that an object should not be mutating the parts of an acquaintance's parts, but should ask the acquaintance to do so. I'd say that a.b.c.d = val is at the very least a sign that some encapsulation did not happen.

Absolutely! But in Haskell how do you do the asking? I guess that's what I'm proposing is a built in way of doing just that! I'm shooting for as-easy-as the built in getters.

Erik Hesselink said:

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Isn't this exactly the problem solved by all the lens packages?

Yes it is. I think the existence of these packages along with all the proposals to change records is an indication that something is missing from the language as a whole. What I'm proposing is that the language give you something that is lightweight and easy to use to address this issue. You can still use lenses on top of all of this.

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makeLens myField myField'

If I remember correctly, one of the problems with lenses is that they cannot support polymorphic updates (updates which change a type variable of the data). SEC functions, on the other hand support polymorphic updates.

--Jonathan

On Thu, Aug 2, 2012 at 4:48 AM, Andrew Butterfield wrote:

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Ah yes - the joy of Haskell

It so easy to roll your own, rather than search to find someone else's (better/more elegant) solution... :-)

On 2 Aug 2012, at 11:41, Erik Hesselink wrote:

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On Thu, Aug 2, 2012 at 12:30 PM, Andrew Butterfield wrote:

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On 2 Aug 2012, at 09:25, Erik Hesselink wrote:

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Isn't this exactly the problem solved by all the lens packages? Current popular ones are fclabels [0] and data-lens [1].

[0] http://hackage.haskell.org/package/fclabels [1] http://hackage.haskell.org/package/data-lens

Not sure what all of these do, but I have a simple solution I use in my work:

They do exactly that. They create 'lenses' which are getters/setters/modifiers combined, and allow you to compose these to get/set/modify deep inside nested data types. Look at the examples in the fclabels documentation [2] for more details.

[2] http://hackage.haskell.org/packages/archive/fclabels/1.1.4/doc/html/Data-Lab...

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