[Haskell-cafe] Distinct types in a list
ozgurakgun at gmail.com
Thu Jan 7 13:44:41 EST 2010
If I understand you correctly, what you want is very similar to catMaybes
isSuccess' (Success a) = Just a
isSuccess' _ = Nothing
result = catMaybes $ map isSuccess ps
This should do the trick.
2010/1/7 rodrigo.bonifacio <rodrigo.bonifacio at uol.com.br>
> 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,
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
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