[Haskell-cafe] Re: Distinct types in a list
Christian.Maeder at dfki.de
Thu Jan 7 14:02:15 EST 2010
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.
> 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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