Having trouble understanding Haskell's type system
algebraic-data-types, constructor, haskell, overloading, types
Solution
The problem you are having is that the Either datatype does not have a data constructor called Either, basically the Either type looks like this
data Either a b = Left a
| Right b
So a value can have the type `Either a b` , but there is no value like `Either "one" 1` or something like that, but instead `Left "one"`, or `Right 1`.
So in the case of `EitherLeft`, similarly its values will look like `EitherLeft (Left a)` or `EitherLeft (Right b)`, and need to be pattern matched against as such.
Problem
I am currently trying to do 20 Intermediate Haskell Exercises. I was able to get done with the 1st 3 exercises (but this is because `furry` == `fmap` and Learn You a Haskell has those implementations already). I am currently stuck on the instance that says: ``` instance Fluffy (EitherLeft t) where furry = error "todo" ``` I am not really understanding what to do. In Learn You Haskell they have a `newtype` variable called `Pair` which takes in a tuple. They then can do pattern matching as such: ``` fmap f (Pair (x,y)) = Pair (f x, y) ``` I was thinking maybe you could do something similar in my situation: ``` furry f (EitherLeft (Either a b)) = EitherLeft (Either (f a) b) ``` But, that doesn't work: ``` Not in scope: data constructor `Either' ``` I was thinking maybe I would `import Data.Either` because there might be some import things he has I don't have. But that didn't matter. I also tried to get just this to work: ``` furry f (EitherLeft a b) = error "todo" ``` But that doesn't work either: ``` Constructor `EitherLeft' should have 1 argument, but has been given 2 ``` I couldn't get this to work either: ``` furry f (Right x) = (Right f x) furry f (Left x) = Left x ``` Which gave the error: ``` Couldn't match expected type `EitherLeft t a' with actual type `Either t0 t1' ``` I have only been able to get: ``` furry f (EitherLeft t) = error "todo" ``` to work. But I have no idea what to do with `t`. I don't necessarily want an answer. I just need a hint as to what to do because I'm reading and I can sort of, understand the examples but I can't really get my head around to coding this stuff right on my own. Thanks Dan, this is what I came up with for my solution: ``` instance Fluffy (EitherLeft t) where furry f (EitherLeft (Left x)) = EitherLeft $ Left (f x) furry f (EitherLeft (Right x)) = EitherLeft $ Right x ```