On Tue, Jul 31, 2012 at 1:13 PM, Alexander Foremny
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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I think you might be looking for something like vault[1]. HTH, Michael [1] http://hackage.haskell.org/package/vault