CODEWITHSUNDEEP
haskell
Vinny
Vinny

Reputation: 134

get element of datatype with id in haskell

Say I have two datatypes look like this:

data DataType1 = DataType1 { id :: Int, values :: [Int]} deriving (Show)
data DataType2 = DataType2 { dataType1Id :: Int, values2 :: [Int]} deriving (Show)

Now I want to compare the average of values and the related values2 (so where DataType1Id = id). I am pretty new in Haskell so I don't know if this is possible.

This is my function to compare the averages:

isGreaterAvgThen :: DataType2 -> Bool
isGreaterAvgThen x
    | average (values2 x) > average (values y) = True
    | otherwise = False
    where y = ...

average is a function that calculates the average of a list of Ints (average :: [Int] -> Double)

Upvotes: 0

Views: 243

Answers (1)

jamshidh
jamshidh

Reputation: 12070

Would this work for you?

where y = fromMaybe (error "missing value") $ 
                     find ((== id x) . dataType1Id) listOfDataType2

Since the list might be missing the desired value, the type of find is wrapped in Maybe.... If you are sure that it is there, you can unwrap it with fromMaybe like I did, but be warned, error is just another way to say "crash"....

Upvotes: 2

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