Re: [Haskell-cafe] matching constructors
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) ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) smime.p7s Description: S/MIME cryptographic signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
Vadim Zaliva wrote: [...] It slightly bothers me that this solution seems to be using non-standard GHC extensions. Hmmm, using generics seems like massive overkill for option handling. Could you describe what you are exactly trying to achieve? Cheers, S. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
On Mar 8, 2004, at 11:17, Sven Panne wrote: Hmmm, using generics seems like massive overkill for option handling. Could you describe what you are exactly trying to achieve? I am doing command line options parsing. I've defined Flag type with constructor for each possible option: data Flag = Verbose | Input String | Output String | Filter String deriving (Show, Typeable, Data) getOpt returns me a list of such objects. Now I need to look things up there by constructor. For example: doSomething fltflag where (Filter fltflag) = findFlag (Filter none) opts To achieve this I've defined: instance Eq Flag where x == y = toConstr x == toConstr y findFlag :: Flag - [Flag] - Flag findFlag f fs = fromMaybe f (find (==f) fs) Sincerely, 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) smime.p7s Description: S/MIME cryptographic signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
On Mon, 8 Mar 2004, Vadim Zaliva wrote: I am doing command line options parsing. I've defined Flag type with constructor for each possible option: data Flag = Verbose | Input String | Output String | Filter String deriving (Show, Typeable, Data) getOpt returns me a list of such objects. Now I need to look things up there by constructor. For example: doSomething fltflag where (Filter fltflag) = findFlag (Filter none) opts Try this instead: doSomething $ option none [fltflag | Filter fltflag - opts] ... option :: a - [a] - a option def [] = def option def [x] = x option def _ = error Only one of each option allowed -- Ben ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
On Mar 8, 2004, at 12:55, Ben Rudiak-Gould wrote: This would work, but I will have to write [] part for each option. Generics approach is overkill but looks much neater when used. But thanks for suggestion anyway, it is always good to learn yet another way of doing things. Sincerely, Vadim Try this instead: doSomething $ option none [fltflag | Filter fltflag - opts] option :: a - [a] - a option def [] = def option def [x] = x option def _ = error Only one of each option allowed -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) smime.p7s Description: S/MIME cryptographic signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
