Haskell Correctly use Multiple Parameters

Question

I am trying to make my own "Set" datatype and when I try to declare an insert function the compiler complains about too few arguments:

quantities.hs:12:27:
Expecting one more argument to `Set'
In the type signature for `insert': insert :: Set e -> Int -> Set

How can I correctly define the insert function to add a new element to my set?

Here is what I have so far:

data Set e = Set [e]

mySet :: Set Int
mySet = Set [1, 2, 3, 4, 5]

setLength :: Set e -> Int
setLength (Set s) = length s

empty :: Set e -> Bool
empty (Set s) = if null s then True else False

insert :: Set e -> Int -> Set
insert set value = set : value

main = do print(insert mySet 1)

About the error itself: Set is not a type, Set e is a type. You forgot the e in the signature of insert. — chi
– chi, Commented May 30, 2015 at 16:16
empty should be written empty (Set s) = null s. if boolean then True else False is exactly the same as just boolean. — Jedai
– Jedai, Commented May 30, 2015 at 19:37

Sibi · Accepted Answer · 2015-05-30 16:19:36Z

3

You have to implement insert like as follows. Note that your type signature isn't correct. You cannot insert Int to a set of type Set e (you can only insert Int to a set of type Set Int).

insert :: Set e -> e -> Set e
insert (Set s) value = Set $ value : s

Note that the above insert doesn't take into account of duplicate elements(Hint: use nub for eliminating that).

edited May 30, 2015 at 16:19

answered May 30, 2015 at 16:00

Sibi

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2 Comments

Samuel Barnes Over a year ago

I get this: Couldn't match type [e0]' with Set e0' Expected type: [Set e0] Actual type: [e] In the second argument of (:)', namely s' In the expression: Set value : s In an equation for `insert': insert (Set s) value = Set value : s

Sibi Over a year ago

@gravysam I think you are missing $according to the error report ? Also see this: ideone.com/3LpTPq

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