[Haskell-cafe] Distinct types in a list

rodrigo.bonifacio rodrigo.bonifacio at uol.com.br
Thu Jan 7 13:35:10 EST 2010

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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