Confusion about custom data types and Haskell

Question

data Quant = Single | Multiple deriving (Show, Read, Eq)

data ListUnit = ListUnit {
    quant :: Quant,
    num :: Int,
    letter :: Char
    } deriving (Show, Read, Eq)

data ListUnit2 = ListUnit2 Quant Int Char deriving (Show, Read, Eq)

decodeModified :: Quant -> Int -> Char -> Char
decodeModified _ _ ls = ls

decodeModified2 :: ListUnit2 -> Char
decodeModified2 _ _ ls = ls

I'm new to data types in Haskell. I'm wondering why ListUnit works but I'm having problems with ListUnit2. I know I shouldn't have to break out all of ListUnits values like I do in the first instance. But when I don't, GHC that I'm not passing enough arguments. I don't see how what I'm doing is different than LYAH in the Shapes example .

In case anyone is wondering, this is for "99 Problems" #11. The data I want to model is [Multiple 4 'a',Single 'b',Multiple 2 'c', Multiple 2 'a',Single 'd',Multiple 4 'e'] .

Answer 1

The problem is that this:

decodeModified2 :: ListUnit2 -> Char

means " decodeModified2 takes a ListUnit2 _(type) and returns a Char ", whereas this:

decodeModified2 _ _ ls = ls

means " decodeModified2 takes three arguments, discarding the first two and returning the third". These two statements are incompatible.

I think what you meant to write is:

decodeModified2 (ListUnit2 _ _ ls) = ls

which means " decodeModified2 takes a value constructed by ListUnit2 _(function) with three arguments, discarding the first two and returning the third".

Does that make sense?

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Edited to add after your comment below: And similarly, when you call decodeModified2 , you need to pass an instance of ListUnit2 _(type) . For example, you can write something like decodeModified2 (ListUnit2 3 's' 'x') , using the value constructor ListUnit2 _(function) to create an instance of ListUnit2 _(type) . (Or, more interestingly: you can create an instance of ListUnit2 _(type) in one place and then call decodeModified2 in a different place, passing said instance.)