CODEWITHSUNDEEP
haskelltuples
mrQWERTY
mrQWERTY

Reputation: 4149

Deleting element from a tuple's list

I am having trouble writing a function which deletes an element from a list within a tuple.

The problem is, I would like the function to return the tuple. However, using the delete function from Data.List gives me a list.

Code:

-- type Value = Int
type Cell = (Int, [Int])
type Board = [Cell]

----------- your solution goes here --------------


-- solvem :: Board -> Board
-- solvem bd = ???
deleteCandidate :: Int -> Cell -> Cell
deleteCandidate cand c1 = delete cand (snd c1) -- This gives me trouble

updateNeighbors :: Cell -> Cell -> Cell
updateNeighbors c1 c2 | isNeighbor c1 c2 = deleteCandidate (head (snd c1)) c2
                      | otherwise = c2

Since data in Haskell is immutable, how would I return a tuple in deleteCandidate function? Would I have to reconstruct a Cell?

Upvotes: 1

Views: 280

Answers (1)

amalloy
amalloy

Reputation: 91907

Simply pattern-match the tuple to extract the two pieces, operate on one of them, and then put it back together into a new tuple:

deleteCandidate :: Int -> Cell -> Cell
deleteCandidate cand (x, xs) = (x, delete cand xs)

Alternatively, since (a,) has a Functor instance, you could write 

deleteCandidate :: Int -> Cell -> Cell
deleteCandidate cand = fmap (delete cand)

Or, shorter still, you could elide the explicit cand argument as well, and express deleteCandidate as a simple function composition:

deleteCandidate :: Int -> Cell -> Cell
deleteCandidate = fmap . delete

Personally I like the second version best, because I find it hard to think about how this composition works, and how it's different from (fmap delete).

Upvotes: 6

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