Group matches in case expression

Question

I have a variable like

a = 3

For some values of a, I have the same function I want to call:

case a of
     3 -> print "hello"
     4 -> print "hello"
     5 -> print "hello"
     6 -> print "hello something else"

So for a=3, a=4 and a=5 I make the same function call. Can I group these better? I'm kinda looking for a solution which would be:

case a of
     3 || 4 || 5 -> print "hello"
     6           -> print "hello something else"

This doesn't work of course, but hopefully you get where I want to end up with.

Thanks.

Answer 1

How about

case a of
     _ | a == 3 || a == 4 || a == 5
       -> print "hello"
     6 -> print "hello something else"

Less tedious to write would be

case a of
     _ | a `elem` [3, 4, 5]
       -> print "hello"
     6 -> print "hello something else"

or

case a of
     _ | 3 <= a && a <= 5
       -> print "hello"
     6 -> print "hello something else"

or even, if in your real program there were a lot of possible values for you to match against, something like this:

import qualified Data.Set as S

valuesToMatchAgainst :: S.Set Int
valuesToMatchAgainst = S.fromList [3, 4, 5]

-- ...

    case a of
         _ | a `S.elem` valuesToMatchAgainst
           -> print "hello"
         6 -> print "hello something else"

(I'm presuming you understand already that _ is a wildcard that matches any value, and that | introduces a guard.)

Answer 2

You can do different things to improve your code. First, if all branches call the same function then why not:

print (case a of 
         3 -> "hello"
         4 -> "hello"
         5 -> "hello"
         6 -> "hello something else")

This factors out more of the common behaviour of your code. Second, you seem to ask about grouping the 3,4 and 5 cases together, the best way might be to factor out a categorization function:

 let cat 3 = True
     cat 4 = True
     cat 5 = True
     cat 6 = False
 in print (case cat a of True -> "hello"
                         False -> "hello something else")

You can combine this with one of the alternatives suggested by the previous poster (cat x = xelem[3,4,5] etc).