Kernel.--/2 weird behaviour

++ and -- are right-associative operations. Your initial code:

[:foo, :bar] -- [:foo] -- [:foo, :bar]

Is actually evaluated as

[:foo, :bar] -- ([:foo] -- [:foo, :bar])

When you take what's in the brackets: you remove a list of elements from [:foo], and the list you remove contains the original list ([:foo]) thus evaluates to an empty list [].

Then you remove this empty list from the leftmost list:

[:foo, :bar] -- []

Which leaves you with the result [:foo, :bar].