[Haskell-cafe] Key-Parametrized Lookup Table

[Michael Snoyman]

On Tue, Jul 31, 2012 at 1:13 PM, Alexander Foremny
<alexanderforemny at gmail.com> wrote:
> 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

I think you might be looking for something like vault[1].

HTH,
Michael

[1] http://hackage.haskell.org/package/vault