The empty list identifier

Our parser recognizes the empty list ‘[]’ as an identifier:

>>> from xotl.fl.parsers.expressions import parse as expr_parse
>>> expr_parse('[]')
Identifier('[]')

Having ‘[]’ as an Identifier could be confusing. It’s a constant. It should be a Literal.

However, there’s no way we can construct a Literal for it (unless we construct many types). The type of ‘[]’ is actually polymorphic (forall a. [a]). The following expressions are both valid:

• 1:[]
• 'a':[]

Furthermore, if we were to create a program:

data List a = Nil | Cons a (List a)

reverse Nil = Nil
reverse (Cons x xs) = append (reverse xs) (Cons x Nil)

Our Nil is a data constructor with no argument (i.e an identifier) and it’s type scheme is forall a. List a.

So why, [] should be different than Nil?