Haskell grouping list elements by fst

Question

I'm trying to write a function

group::[(Int, Int)]->[[(Int, Int)]]

that would group elements of a list of tuples into subgroups dependant on the first coordinate, ie:

group [(1,1),(1,2),(2,1),(2,2),(2,3)]

should result in

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

I'd imagine list comprehension is the way, but I'm a bit stuck.. Could someone provide an advice/solution?

PS the function could also take as an argument the maximum the first coordinate can take, if that would be of any help

Answer 1

You can do this using groupBy

λ: groupBy (\x y -> fst x == fst y) [(1,1),(1,2),(2,1),(2,2),(2,3)]
[[(1,1),(1,2)],[(2,1),(2,2),(2,3)]]

The on function is useful here to shorten the code a bit without obfuscating it:

λ: groupBy ((==) `on` fst) [(1,1),(1,2),(2,1),(2,2),(2,3)]
[[(1,1),(1,2)],[(2,1),(2,2),(2,3)]]