Defining a function in Haskell that returns true if it is the list consisting of 'a', false otherwise

Defining a function in Haskell that returns true if it is the list
consisting of 'a', false otherwise

I'm new to Haskell, and I'm trying to write a function that takes a list
and returns a bool.
It will return True if its input list is the list consisting of 'a' only,
and False otherwise.
This is my best guess:
f :: [a] -> Bool
f ('a':[]) = True
f (x:xs) = False
This fails to compile and returns:
Couldn't match type `a' with `Char'
`a' is a rigid type variable bound by
the type signature for f :: [a] -> Bool at charf.hs:6:1
In the pattern: 'b'
In the pattern: 'b' : []
In an equation for `f': f ('b' : []) = True
What is the error in my logic?