Hi,
using list comprehensions for the kind filter&map operation you're
wanting to express, seems like a good option.
border p = check "no border in page" [ x | (Border x) <- p]
margin p = check "no margin in page" [ x | (Margin x) <- p]
check :: String -> [a] -> a
check err_msg [] = error err_msg -- game over.
check err_msg (x:_) = x
--Sigbjorn
S. Alexander Jacobson writes:
> I am building a description of page state using a list of pageStyles
> > data PageStyle= Orientation Orient
> > | Margin BoxData
> > | Border BoxData
> > | CurrentYPos Float
> > | CurrentXPos Float
>
> To extract particular items from the current pagestate, I create functions
> of the form:
>
> > border [] = error "no border in page"
> > border (a:xs) = case a of
> > Border x -> x
> > otherwise -> border xs
>
> It would be much nicer if I could create a generalized form of this like:
> > border p = extract Border "no border in page" p
>
> But the extract function appears to be illegal:
> > extract f s [] = error s
> > extract f s (a:xs) = case a of
> > f x -> x
> > otherwise -> extract f s xs
>
> Is there another way of getting this functionality?
>
> -Alex-
>
> ___________________________________________________________________
> S. Alexander Jacobson i2x Media
> 1-212-697-0184 voice 1-212-697-1427 fax
