CODEWITHSUNDEEP
haskellghcghci
SilentDev
SilentDev

Reputation: 22747

Checking multiple conditions in a case

Suppose I have this (inserting into a BST):

mybstInsert k inp@(INode left key right)
    | k < key = myNode (mybstInsert k left) key right
    | k > key = myNode left key (mybstInsert k right)

What I want is, in the second condition, I want another check as well (checking if right is empty or not). So I want to break the second condition down into two conditions:

| k > key && isEmpty right == True = myNode left key (mybstInsert k right)
| k > key && isEmpty right == False = myNode left key (mybstInsert k right)

Assuming I have an isEmpty function that takes a subtree and tells me if it is empty or not.

What would be the recommended way to do this? Is this the correct syntax? (I'm working on a computer that does not have GHCi installed so I cannot check the syntax.

Upvotes: 0

Views: 93

Answers (1)

Fyodor Soikin
Fyodor Soikin

Reputation: 80724

This is the correct syntax, but you can also simplify it a bit.

First, observe that the expression x == True is equivalent to just x. So you can drop the == True.

Second, the == False part is not necessary at all. This is because by the time you get around to checking that case, it is already known that isEmpty right == False, because if it wasn't, you would have stopped on the previous case.

So, the whole thing can be written like this:

mybstInsert k inp@(INode left key right)
    | k < key = myNode (mybstInsert k left) key right
    | k > key && isEmpty right = myNode left key (mybstInsert k right)
    | k > key = myNode left key (mybstInsert k right)

Finally, I'd like to point out that you're not handling the case of k == key, so you program would crash on such input. If you wanted to include that case (and I'm assuming it would just be a no-op), then you don't have to include even the k > key check, because by that time it would already be known that neither k == key nor k < key is true:

mybstInsert k inp@(INode left key right)
    | k == key = inp
    | k < key = myNode (mybstInsert k left) key right
    | isEmpty right = myNode left key (mybstInsert k right)
    | otherwise = myNode left key (mybstInsert k right)

Upvotes: 2

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