[Haskell-beginners] How to correctly define data types?

[Brent Yorgey]

On Mon, Jul 18, 2011 at 02:26:33PM -0300, Davi Santos wrote:
> Hello (first post),
> I have spent so many time learning typeclasses and thinking it was part of
> Haskell essence... and suddenly I read the discussion "[Haskell-beginners]
> Can fields in a record be optional?".
> So typeclasses are not always recomended?
> 
> I'm implementing a Machine Learning framework and I am in a sort of related
> dilemma.
> 
> I found three ways of implementing the same distance function between
> "examples" (aka "attribute vectors" or simply "Float vectors" for mere
> mortals :) ):
> 
> [obs: "Example" datatype will be added more fields later]
> 
> --------------first------------------------------------
> module ML where
> 
> data Example =
>      Example [Float] deriving (Show)
> 
> class ExampleClass a where
>      (distance) :: a →  a →  Float
> 
> instance ExampleClass Example where
>    (Example atts1) distance (Example atts2) =
>         sqrt $ sum $ map (λ(x, y) →  (x-y)↑2) $ zip atts1 atts2
> =================================

This is an unnecessary use of type classes, unless you plan to make
additional instances of ExampleClass later.

> --------------second------------------------------------
> module ML where
> 
> data Example =
>     Example {attributes :: [Float]} deriving (Show)
> 
> distance :: Example →  Example →  Float
> distance ex1 ex2 =
>         sqrt $ sum $ map (λ(x, y) →  (x-y)↑2) $
>                 zip (attributes ex1) (attributes ex2)
> =================================
> 
> 
> --------------third------------------------------------
> module ML where
> 
> data Example =
>        Example [Float] deriving (Show)
> 
> distance :: Example →  Example →  Float
> distance (Example att1) (Example att2) =
>       sqrt $ sum $ map (λ(x, y) →  (x-y)↑2) $
>                 zip (att1) (att2)
> =================================

I'd say these are about the same style-wise, it's a matter of
preference.   But if more fields will be added to Example later, using
record labels may be a good idea.

> All three reserves the word "distance" for itself and the second reserves
> also the word "attributes".
> How could I implement the module ML and which would be the best way  to set
> "attributes" outside the module?

I'm not sure what you mean by "set 'attributes' outside the module",
can you clarify?

-Brent