[Haskell-cafe] matching constructors

Vadim Zaliva lord at crocodile.org
Fri Mar 5 15:48:25 EST 2004

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.


Thanks for great suggestions! Following them, here is how I redone the 


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 -> 
3. inline lambda expression passed to find

I am leaning towards #2 or #3.


"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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