Re[2]: [Haskell-cafe] Re: Distinct types in a list

Hello Keith, Thursday, January 7, 2010, 11:26:51 PM, you wrote: existential types has more limited usage compared to dynamics, but in the cases they work they allow to have compile-time type-checking instead of run-time one

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?

Best Keith

On Thu, Jan 7, 2010 at 2:02 PM, Christian Maeder wrote:

...

You could cast your parser result "a" to Dynamic using Data.Dynamic.toDyn (and derive Typeable instances for all involved types).

http://www.haskell.org/ghc/docs/latest/html/libraries/base-4.2.0.0/Data-Dyna...

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,

Rodrigo.

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