This snippet is from LYAH:
instance (Eq m) => Eq (Maybe m) where
Just x == Just y = x == y
Nothing == Nothing = True
_ == _ = False
I am totally confused as to what x and y are supposed to be since they are not defined anywhere. Can anyone help me understand this?
Well, == is an infix function, you can read this as a pattern matching on:
(==) (Just x) (Just y)
In which case it is clear that x and y are the function parameters in the pattern matching.
A simpler example can be shown (without type classes) as:
areBothFive:: Int -> Int -> Bool
5 `areBothFive` 5 = True
x `areBothFive` y = False -- the x and y are variables in the pattern match here
areBothFive 5 5 -- true
areBothFive 4 5 -- false
Here's a fiddle illustrating the issue.
LYAH gives an example using this syntax in the "Syntax in Functions" chapter.
They are there to deconstruct the pattern Just _ - so:
let (Just x) = Just 5 will give you x <- 5 let (Just x) = Nothing will just match (and fall through to another case - here it will be the _ == _the let is just there so that you can try this out in GHCi:
> let (Just x) = Just 5
> x
5
> let (Just x) = Nothing
> x
*** Exception: <interactive>:4:5-22: Irrefutable pattern failed for pattern (Just x)
