CODEWITHSUNDEEP
haskell
peroni_santo
peroni_santo

Reputation: 367

Pattern match list elements

Is there a way to look through a list and when you find the values 4 and 5, do something?

I tried foo (4:5:xs) = <do something> but it doesn't compile

Upvotes: 1

Views: 710

Answers (1)

yatima2975
yatima2975

Reputation: 6610

You'll also have to describe what should happen when you don't find 4 and 5!

Let's suppose that you want to return the string "Found" when you have seen 4 and 5, and "Not found" otherwise. Then you could use this function:

foo :: [Int] -> String
foo (4:5:xs) = "Found"
foo (_:xs) = foo xs
foo [] = "Not found"

If you don't want to 'do anything' when you do not see 4 and 5, you'll have to change the return type of function to Maybe String (in this example):

foo :: [Int] -> Maybe String
foo (4:5:xs) = Just "Found"
foo (_:xs) = foo xs
foo [] = Nothing

I would use the second version, so that you don't have to remember what the 'not found' value is.

Upvotes: 5

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