How to find duplicates in a list of tuples?

Question

What would be the best way to filter through a list of tuples and return only the ones where the fst and snd value are the same ?

[(2,1),(2,2),(3,1),(10,9),(10,10)] 

would return (2,2) and (10,10).

Answer 1

The easiest way is to just use filter with a lambda: filter (\ (a, b) -> a == b) ls.

You could also be cute and use uncurry, which changes a normal function of two arguments into one that takes a tuple, giving you filter (uncurry (==)). Remember that (==) is just a function of type Eq a => a -> a -> Bool, so uncurry (==) is a function of type Eq a => (a, a) -> Bool, which is exactly what you're looking for.

Answer 2

You can do it using list comprehensions:

doubles ls = [(x,y) | (x,y) <- ls, x==y]