[Haskell-cafe] Key-Parametrized Lookup Table

[Paolino]

This is without class :-)

{-# LANGUAGE TypeFamilies, ExistentialQuantification, FlexibleContexts #-}
import Prelude hiding (lookup)
import Data.Typeable

type family Value a :: *

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

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

lookup :: (Typeable (Value a), Typeable a, Eq 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

*Main> type instance Value Integer  = Char
*Main> type instance Value Int = String
*Main> let u = insert (1::Integer) 'c' $ insert (1::Int) "ciao" []
*Main> lookup (1 :: Integer)  u
'c'
*Main> lookup (1 :: Int)  u
"ciao"
*Main>

Regards
paolino

2012/8/1 Paolino <paolo.veronelli at gmail.com>

>
> 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
>>
>> _______________________________________________
>> Haskell-Cafe mailing list
>> Haskell-Cafe at haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>
>
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