[Haskell-cafe] Re: Distinct types in a list
keithshep at gmail.com
Thu Jan 7 15:26:51 EST 2010
Hello, My impression is that using existential types where possible
will result in more complete type checking than Data.Dynamic but I'm
not sure since I haven't yet tried Data.Dynamic in my own code. Can
someone confirm if this is right?
On Thu, Jan 7, 2010 at 2:02 PM, Christian Maeder
<Christian.Maeder at dfki.de> wrote:
> You could cast your parser result "a" to Dynamic using
> Data.Dynamic.toDyn (and derive Typeable instances for all involved types).
> Using an existential types may be another alternative.
> Cheers Christian
> rodrigo.bonifacio schrieb:
>> Hi all,
>> I have a family of parsers that return either (Success t) or (Fail), using the following data type:
>>> data ParserResult a = Success a | Fail String
>>> deriving (Read, Show, Eq, Ord)
>>> isSuccess (Success _) = True
>>> isSuccess (Fail _) = False
>> I want to add the results of different parsers to a list. Such as:
>>> m1 = parseFirstModel file1 -- it returns a ParserResult of t1
>>> m2 = parseSecondModel file2 -- it returns a ParserResult of t2
>>> ps = [m1, m2]
>> In such a way that I could write something like:
>>> if and (map isSuccess ps)
>>> then process m1 m2
>>> else ...
>> Actually, in the real program I have to check more than two input models. However, since Lists do only hold elements of a same type, I couldn't proceed in this way. Which improvements to the ParserResult data type should I wrote in order to proceed as I want to.
>> Best regards,
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