[Haskell-cafe] Parsing different types, same typeclass

[Jose A. Lopes]

Thanks Stephen, that worked out just fine!

On 18-11-2012 18:23, Jose A. Lopes wrote:
> Thanks Stephen. I will try this!
>
> On 18-11-2012 17:56, Stephen Tetley wrote:
>> With existentials an "extesible" version might look like this:
>>
>>> {-# LANGUAGE ExistentialQuantification #-}
>>> {-# LANGUAGE ScopedTypeVariables #-}
>>
>> ... class Action and datatypes A and B the same as before ...
>>
>>
>>> -- some new ones...
>>> data C = C Int
>>>           deriving (Read, Show)
>>
>>> instance Action C where
>>>       run (C n) = n
>>
>>> data D = D Int
>>>           deriving (Read, Show)
>>> instance Action D where
>>>       run (D n) = n
>>
>> The important one:
>>
>>> data PolyA = forall a. Action a => PolyA a
>>
>>> parseAction :: String -> PolyA
>>> parseAction str
>>>       | "(A " `isPrefixOf` str = PolyA $ (read :: String -> A) str
>>>       | "(B " `isPrefixOf` str = PolyA $ (read :: String -> B) str
>>>
>>>       -- can add new cases
>>>       | "(C " `isPrefixOf` str = PolyA $ (read :: String -> C) str
>>>       | "(D " `isPrefixOf` str = PolyA $ (read :: String -> D) str
>>
>> This is "extensible" to some degree as you can add new cases of
>> "different" types, but all you can do with these different types is
>> run them to make an Int, so it is equivalent to the second version I
>> gave previously:
>>
>>> parseAction :: String -> Int
>>> parseAction str
>>>      | "(A " `isPrefixOf` str = run $ (read str :: A)
>>>      | "(B " `isPrefixOf` str = run $ (read str :: B)
>>>      | "(C " `isPrefixOf` str = run $ (read str :: C)
>>>      | "(D " `isPrefixOf` str = run $ (read str :: D)
>> i.e instead of using an existential type to hold something polymorphic
>> that can be run to produce an Int, just call run at the point of use
>> making an Int.
>

-- 
José António Branquinho de Oliveira Lopes
Instituto Superior Técnico
Technical University of Lisbon