[Haskell-beginners] Case Expressions

[Brent Yorgey]

On Thu, May 28, 2009 at 05:53:43PM -0400, Nathan Holden wrote:
> 
> interToBasic :: Interval -> BasicInterval
> interToBasic a = if (b == 1 || b == 2) then Second
>                           else if (b == 3 || b == 4) then Third
> ..
> where b = fromEnum a
> 
> What I wanted to do, and figure is probably doable, but I can't seem to find
> it, is to say something like
> 
> case (fromEnum a) of
>    1 || 2 -> Second
>    3 || 4 -> Third
> ..
> 
> Is this doable? If so, what's the syntax?

You can do this with pattern guards, like so:

interToBasic a | b == 1 || b == 2  = Second
	       | b == 3 || b == 4  = Third
	       ...
  where b = fromEnum a

You could also use b `elem` [1,2] as an alternative to b == 1 || b ==
2 (especially nice when there are more than two options).

-Brent