[Haskell-cafe] Key-Parametrized Lookup Table

[Alexander Foremny]

Dear Michael,

thank you very much for your quick and interesting response. This
looks very much like what I want!

Regards,
Alexander Foremny

2012/7/31 Michael Snoyman <michael at snoyman.com>:
> 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
>>
>> _______________________________________________
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>> Haskell-Cafe at haskell.org
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>
> I think you might be looking for something like vault[1].
>
> HTH,
> Michael
>
> [1] http://hackage.haskell.org/package/vault