Adding data types in a function

Question

I am new to Haskell and learning about data types. I am attempting to do a simple problem, but I cant seem to grasp the bigger picture. I have a data type of LengthUnit which resembles inches, feet, and yards. I want to make a function that takes a list of lists of LengthUnits and converts them to inches and sums all elements.

I created a helper function convert, which converts the LengthUnit to inches. I also have addLengths which accepts two LengthUnits and adds them up. I cant seem to get the output I am looking for. Here is what I have:

data LengthUnit =  INCH  Int | FOOT  Int | YARD  Int
                   deriving (Show, Read, Eq)

convert :: LengthUnit -> Int  
convert (INCH x) =  (x) 
convert (FOOT x) =  (x * 12)
convert (YARD x) =  (x * 36)

-- addLengths 
addLengths :: LengthUnit -> LengthUnit -> LengthUnit
addLengths x y = INCH ((convert x) + (convert y))


-- addAllLengths
--addAllLengths :: Foldable t => [t LengthUnit] -> LengthUnit
addAllLengths list = let nList = map convert list
                     in foldr addLengths 0 nList

From the above I get an error: Couldn't match type 'Int' with 'LengthUnit'. Any help would be appreciated.

I'm looking to get something like this:

addAllLengths [[FOOT 2], [FOOT 2, INCH 2],[]]
INCH 50

Joseph Sible-Reinstate Monica · Accepted Answer · 2019-09-23 03:42:57Z

3

Try this:

addAllLengths list = foldr addLengths (INCH 0) (concat list)

concat turns [[FOOT 2], [FOOT 2, INCH 2],[]] into [FOOT 2, FOOT 2, INCH 2], and then foldr addLengths (INCH 0) turns that into INCH 50.

answered Sep 23, 2019 at 3:42

Joseph Sible-Reinstate Monica

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1 Comment

Alex Over a year ago

Thank, I didn't know about concat. That would come in handy

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