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