Re: Re: [Haskell-cafe] Type Directed Name Resolution

On Thu, Nov 11, 2010 at 10:00 PM, John Lask wrote:

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On Thu, Nov 11, 2010 at 8:16 PM, John Lask  wrote:

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consider "length" ...

I have records with the attribute length, length can be given as an Int, Double, Float or maybe as a constructed type "Length", length's use as a record selector would also clash with List.length. All these have the same denotation.

should I then seporate into int_length, float_length, or use rec1_length, rec2_length etc etc...

class Lengthy a where   type LengthType a   length :: a ->  LengthType a

This extends easily to lenses if you want setters.

to make use of the class Lengthy I still need to define record selectors with different names, which is exactly the point I am making...

....ie

data Record = RecLen { rec_length :: ... }

instance Lengthy Record where  length = rec_length

Or, data Record = RecLen Int ... instance Lengthy Record where type LengthType Record = Int length (Record l _ _ ...) = l But yes, this is irritating boilerplate. A more-powerful record system is clearly preferable.

The point is that languages are often constructed with a purpose in mind, at which they tend to be particularly good. Haskell has traditionally been a test bed for type system innovation, which is why we all like it so much.

Which is why I'm opposed to TDNR. -- Dave Menendez http://www.eyrie.org/~zednenem/