[Haskell-cafe] Seeking for an extention (functional incapsulation)

[Dan Weston]

Is there any good extension? Yes, it's in Control.Applicative.



Belka wrote:
> Hello, cafe visitors! :)
> 
> This is a double topic:
> 1. Can't find any good informative resource with descriptions of Haskell
> extensions. Could anybody please share good one if it exists?
> The only "good" one I found:
> http://hackage.haskell.org/trac/haskell-prime/wiki/HaskellExtensions
> But it's a bit too old and not that full... 
> I undestand, that Haskell is kind of "boiling" language, in a sense of being
> neverending experiment. It develops all the time, extensions show up and
> drop out. So it's not that easy to support community with a fresh
> information about them. But on the other side, the property (of being
> "boiling" language) makes such information really important for community
> members... I think. :)
> 
> 2. Consider situation:
> -----------------------------------
> data SomeDataType = SomeDataType { sdtField1 :: SDT_Field1Type, sdtField2 ::
> SDT_Field2Type}
> sdtField3 :: SomeDataType -> SDT_Field3Type
> sdtField3 sdt = f (sdtField1 sdt) (sdtField2 sdt)
> -----------------------------------
> I induced recently, that it would be very comfortable if I could perform in
> a way like this:
> -----------------------------------
> data SomeDataType = SomeDataType { sdtField1 :: SDT_Field1Type, sdtField2 ::
> SDT_Field2Type, sdtField3 :: SDT_Field2Type, sdtField3 = f sdtField1
> sdtField2}
> -----------------------------------
> The situation is not that rare, when dealing with nonprimitive data
> constructions. Moreover would be really comfortable to reduce
> -----------------------------------
> data SomeDataType = SomeDataType_111 { sdtField1 :: SDT_Field1Type,
> sdtField2 :: SDT_Field2Type} | SomeDataType_222 { sdtField1 ::
> SDT_Field1Type, sdtField2 :: SDT_Field2Type, sdtField5 :: SDT_Field5Type}
> 
> sdtField3 :: SomeDataType -> SDT_Field3Type
> sdtField3 sdt = case sdt of {SomeDataType_111 -> f (sdtField1 sdt)
> (sdtField2 sdt) ; SomeDataType_222 -> g (sdtField1 sdt) (sdtField2 sdt)
> (sdtField5 sdt)}
> 
> \/ \/ \/ \/ \/ \/ \/ \/ \/ \/
> 
> data SomeDataType = SomeDataType_111 { sdtField1 :: SDT_Field1Type,
> sdtField2 :: SDT_Field2Type, sdtField3 :: SDT_Field3Type, sdtField3 = f
> sdtField1 sdtField2} | SomeDataType_222 { sdtField1 :: SDT_Field1Type,
> sdtField2 :: SDT_Field2Type, sdtField5 :: SDT_Field5Type, sdtField3 ::
> SDT_Field3Type, sdtField3 = g sdtField1 sdtField2 sdtField5}
> -----------------------------------
> 
> Usable mechanics for realization would be:
> 1. Funtion similar to Data.Function.on (example: (*) `on` f = \x y -> f x *
> f y), but opposite - I called it under. 
> t `under` f = \x y -> (x f) `t` (y f)
> 2. currying and uncurrying
> 
> Is there any such extension?
> 
> Belka