[Haskell-cafe] Key-Parametrized Lookup Table

Alexander Foremny alexanderforemny at gmail.com
Tue Jul 31 12:13:21 CEST 2012

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.

Alexander Foremny

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