[Haskell-cafe] Key-Parametrized Lookup Table

[Paolino]

Hello, I made some trial and error with ghci to make it happy. I'm not
really sure this has the type safety you asked.

{-# LANGUAGE TypeFamilies, ExistentialQuantification, FlexibleContexts #-}

import Prelude hiding (lookup)
import Data.Typeable

class Typeable a => Key a where
    type Value a :: *

data Assoc = forall a . (Typeable (Value a),Key a) => Assoc a (Value a)

insert :: (Typeable (Value a), Key a) => a -> Value a -> [Assoc] -> [Assoc]
insert k v = (Assoc k v :)

lookup :: (Typeable (Value a), Eq a, Key a) => a -> [Assoc] -> Value a
lookup k [] = error "noassoc"
lookup k ((Assoc k' v):xs) = case cast k' of
        Nothing -> lookup k xs
        Just k'' -> if k'' == k then case cast v of
                Nothing -> error "nocast"
                Just v' -> v'
            else lookup k xs

I've tried without the typeclass with no luck.
For some reasons

type family Key a :: *
type family Value a :: *

and adding Typeable (Key a) to the contexts and Key 'a' in place of 'a'
leads to a lot of type errors.
Maybe it's possible with more help.

Hope I got it right.

Regards
paolino

2012/7/31 Alexander Foremny <alexanderforemny at gmail.com>

> Hello list,
>
> I am currently thinking that a problem of mine would best be solved if
> there was a Map-like data structure in which the value returned is
> parametrized over the lookup type.
>
> I wonder is this makes sense and if such a data structure exists or if
> it could be created while still being well typed. I essentially want
> to statically define a scope of Key values and dynamically define a
> list of keys.
>
> > -- Scope of possible keys.
> > type Label = String
> > data Key a where
> >     KeyStr :: Label -> Key String
> >     KeyInt :: Label -> Key Int
> >     KeyChoice :: Label -> [a] -> Key a
>
> > -- Some key values, to be extended at runtime.
> > strKey "Some String"
> > strKey' "Another String"
> > intKey "Some integer"
> > choiceKey "Chose one" [ "a", "b", "c" ] :: KeyChoice String
>
> Now I need a data structure to possibly associate a value to the key.
>
> > data MapG = ...
> > type Value a = a
> > insert :: Key a -> Value a -> MapG Key Value -> MapG Key Value
> > lookup :: Key a -> MapG Key Value -> Maybe (Value a)
>
> I tried implementing this with multiple Map k a's. I tried adding a
> phantom type on some storage type of to implement KeyChoice as of type
> Key Int, but I ran into troubles with this approach. I wonder if
> Dynamic or Type Families could achieve this, but I am quite at a loss
> and would like to hear your opinion.
>
> I did try to search for this a bit, but I don't quite know how to
> phrase my problem. I'd like to apologize in advance if this question
> has been asked already.
>
> Regards,
> Alexander Foremny
>
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