I'm new to Haskell so I apologize if this is an easy question, but I couldn't find an answer anywhere.
What I would like to do is grab the first and last element of a list and return those in a new list.
I know that head [1,2,3,4] will return 1, and I know that last [1,2,3,4] will return 4. But how would I go about placing those two results into a list such as: [1,4]?
Thanks.
You can place elements into a list using list notation syntax:
firstLast1 xs = [head xs, last xs]
which is syntactic sugar for using the list constructor directly:
firstLast2 xs = head xs : last xs : []
However, both of these will fail with a runtime error when an empty list is passed. You can guard against this by pattern-matching:
firstLast3 [] = []
firstLast3 xs = [head xs, last xs]
firstLast3 [x] = [x].
firstLast3 [x] = [x] or firstLast3 [x] = [x,x]. If the latter, it may also make sense to change the return type to a Maybe [a].Maybe you want to look at Maybe to avoiding empty list case.
headAndLast :: [a] -> Maybe [a]
headAndLast [] = Nothing
headAndLast x = Just [head x, last x]
And
> headAndLast [1,2,3]
Just [1,3]
Num constraint? I also think it would be more useful to have a function headAndLast :: [a] -> Maybe [a]. Composition with sequence may be the most convenient way to do this.
Num constraint is weird; why do you need it? Also, some hacks you may or may not enjoy: lastM = headM . reverse; headAndLast' :: [a] -> Maybe [a]; headAndLast' = liftM2 (\x y -> [x,y]) headM lastM.
liftM2 (\x y -> [x,y]) headM lastM is [Maybe a], but not Maybe [a]
Maybe [a] over Maybe (a,a).[1,4] from [1,2,3,4], not (1,4).
(1,4)instead of[1,4]