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haskelltuples
Thomas
Thomas

Reputation: 251

Haskell: Find tuple with first element x in list of tuples

Allow me to explain if my question is not clear. Let's say I have tuples :: [(a, Int)]. I have created this using zip. Now, I want to find a corresponding Int value for my a element. I'm trying to figure out how I do this.

f :: [(a, Int)] -> a -> Int

Above is the type of the function I need.

Or how it needs to be used in ghci:

let tuples = zip ['a','b','c'] [1..]

f tuples 'a'

returns: 1

Upvotes: 1

Views: 1707

Answers (2)

Mephy
Mephy

Reputation: 2986

As the other answer says, you may wish to use lookup :: Eq a => a -> [(a, b)] -> Maybe b (source-code). You may implement it in point-free style as lookup key = fmap snd . safeHead . filter ((== key) . fst).

To drop the safety when the key is not present on the list, just swap safeHead :: [a] -> Maybe a with head :: [a] -> a, and to change the parameter order simply flip :: (a -> b -> c) -> b -> a -> c.

Therefore, to find your initial specification we could have:

myLookup :: Eq a => [(a, b)] -> a -> b
myLookup = flip myLookup'
    where myLookup' key = snd . head . filter ((== key) . fst)

Upvotes: 1

WhiteViking
WhiteViking

Reputation: 3201

There's a

lookup :: Eq a => a -> [(a, b)] -> Maybe b

in the Prelude. It returns a Maybe because the item you're looking up might not be in the list.

But you didn't ask for safety and the argument order is different from what you asked, so it doesn't answer your question exactly.

Upvotes: 3

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