Empty list type

[rezanayebi]

Apollo now support empty lists. However it should be noted that an empty list is not a list but a type of its own, namely TListEmpty.
This means that a list cannot be defined as empty, e.g.

apollo> a: [Int] = []
Type error: definition of [Integer], but assigned to []

Here, we get a type error as expected.
However, once we cons an element to an empty list, then it turns into a full fledged list type and type checking takes care of verifying if, say, a definition is correct, as in

apollo> b: [Bool] = True :: []
apollo> b
[True]

[benkogan]

Note that there's a special case for using empty list in a case conditional --- see the prelude in Lib. This should be documented.

The return type is whichever part of the case wasn't TListEmpty. If they both are, it'll be TListEmpty.

See the implementation here.