Couldn't match expected type `a' with actual type `[a]'

Question

I was able to execute the following code flawlessly

myLast :: [a] -> a
myLast [] = error "Can't call myLast on an empty list!"
myLast (x:_) = x

but I'm getting this error Couldn't match expected type `a' with actual type `[a]'. `a' is a rigid type variable bound by the type signature for myLast :: [a] -> a for the following code:

myLast :: [a] -> a
myLast [] = error "Can't call myLast on an empty list!"
myLast (_:x) = x

I'm a beginner in Haskell and the error message is too greek and latin for me. From what I can understand, the compiler is not able infer the type in the second case. Can someone point me to what is actually happening here?

Answer 1

You're declaring the input to be a list of type [a], and the rest to be of type a.

A list of type [a] in Haskell consists of a head of type a and a tail, a list of type [a]. The cons constructor : takes the head and tail as its arguments.

When you deconstruct a list as (x:y), x is the head and y is the tail. So in your second code fragment, you're binding the tail of the list, which has the list type [a], when your type signature requires that you return a value of type a (the head being one example).

Answer 2

Having an understanding of what : really is will help decrypt the error message. : can be thought of as a function which takes an element and a list, and returns a list that whose first element is the first argument and the rest of it is the second argument, or:

(:) :: a -> [a] -> [a]

Getting to your function, you wrote myLast :: [a] -> a; however, the type of myLast (_:x) = x is myLast :: [a] -> [a] since the second argument of : (which you named x) is itself a list.

Additionally, in general when you don't understand something in Haskell, you should take a look at it's type first using :t in GHCI.

Answer 3

(_:x) matches _ with the head and x with the tail of the list. The type of tail of a list is [a]. You are trying to return [a]' where as the function declaration specifies return type as a.

myLast (_:x) = x

If you want to match last element take a look at this answer - Can you use pattern matching to bind the last element of a list?