On Fri, 5 Mar 2004, Vadim Zaliva wrote:
Hi!
I am new to Haskell, but I have background in various programming languages (including Lisp)
I have very basic question, if there is a way to match algebraic types constructors besides use of pattern matching. I wrote today code like this:
data Flag = Verbose | Input String | Output String | Filter String deriving Show
instance Eq Flag where Verbose == Verbose = True Input _ == Input _ = True Output _ == Output _ = True Filter _ == Filter _ = True _ == _ = False
findFlag :: Flag -> [Flag] -> Flag findFlag f [] = f findFlag y (x:xs) = if x == y then x else findFlag y xs
If there is a cleaner way to implement findFlag and Eq Flag?
Vadim
It looks like you are doing some sort of option processing. There have been some suggestions recently for better approaches on the haskell list. See the thread "High-level technique for program options handling" from January. The basic idea there is to define a record type holding your options in the most useful form for your program, and turn your arguments into functions that transfrom such a record, rather than values you need to analyze. The example in Tomasz Zeilonka's post has exactly the option structure of your code. At the lower level of remimplementing your functions I can suggest a few things. It's probably overkill, but the Data typeclass provides an operation that takes a value and returns the constructor used. Add "Data" to the list of typeclasses you want to derive, and you can write your equality function as (GHC only) x == y = toConstr x == toConstr y For your findFlag function, the library function "find" is nearly what you want, except it returns a value in Maybe rather than taking a default: findFlag :: Flag -> [Flag] -> Maybe Flag findFlag f fs = find (==f) fs You could use the "maybe" function (Maybe a -> b -> (a->b) -> b) to supply the default. Brandon
-- "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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