[Haskell-cafe] matching constructors

[Vadim Zaliva]

On Mar 5, 2004, at 15:48, Vadim Zaliva wrote:

OK, I figured it out. For sake of other novices like me here is what 
you need
to do to make it work.

0. Need to import Data.Generics
1. Compile with '-fglasgow-exts' flag
2. When deriving from Data you also need to derive from Typeable.

It slightly bothers me that this solution seems to be using 
non-standard GHC extensions.

Vadim

>
> On Mar 5, 2004, at 12:41, Brandon Michael Moore wrote:
>
>> At the lower level of remimplementing your functions I can suggest a 
>> few
>> things.
>
> Brandon,
>
> Thanks for great suggestions! Following them, here is how I redone the 
> code:
>
> ...
>
> import Data.Maybe
> import Data.Either
> import Data.Typeable
>
> ...
>
> data Flag = Verbose |
>             Input String  |
>             Output String |
>             Filter String
>             deriving Show Data
>
> instance Eq Flag where
>     x == y = toConstr x == toConstr y
>
> findFlag :: Flag -> [Flag] -> Flag
> findFlag f fs = fromMaybe f (find (==f) fs)
>
> The only problem is my GHC does not seems to find 'Data' class and I 
> am getting following errors:
>
> Type constructor or class not in scope: `Data'
> Variable not in scope: `toConstr'
> Variable not in scope: `toConstr'
>
> Also I have style question: What is the best way to define equality 
> test in this example:
>
> 1. Via instantiating EQ class
> 2. via standalone function (I can define sameConstr Flag -> Flag -> 
> Bool)
> 3. inline lambda expression passed to find
>
> I am leaning towards #2 or #3.
>
> Vadim
>
> --
> "La perfection est atteinte non quand il ne reste rien a ajouter, mais
> quand il ne reste rien a enlever."  (Antoine de Saint-Exupery)
>
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--
"La perfection est atteinte non quand il ne reste rien a ajouter, mais
quand il ne reste rien a enlever."  (Antoine de Saint-Exupery)

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