[Haskell-cafe] matching constructors

[Ben Rudiak-Gould]

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