[Haskell-beginners] Pattern Matching

[Tillmann Rendel]

Alex Watt wrote:
> The best way I can explain what I'd like to do is show an example, so
> here goes:
> 
> same :: a -> a -> Bool 
> same x x = True 
> same _ _ = False
> 
> Is there any way to do this in Haskell?

No, this is not possible. Each variable bound in a pattern has to be
different. Maybe guards are a good alternative:

   same :: Eq a =>  a -> a -> Bool
   same x y | x == y = True
   same _ _ = False

Note that you have to add an Eq constraint.

> The actual reason I'd like to do this is something slightly more
> complicated. I want to match against a list of elements, like so: 
> 
> foo[x,x,x] = ...

Maybe you can employ the "all" function

   all :: (a -> Bool) -> [a] -> Bool

in a guard, something like

   foo (x:xs) | all (x ==) xs = ...

where you explicitly check that all list elements are equal to the first.

   Tillmann