CODEWITHSUNDEEP
haskell
Chris
Chris

Reputation: 1128

Confusion about custom data types and Haskell

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'].

Upvotes: 0

Views: 87

Answers (1)

ruakh
ruakh

Reputation: 183211

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?

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.)

Upvotes: 4

Related Questions