I think such a construct would be more confusing than useful.
Consider this piece of code:
if my_condition then
if my_other_condition then
If we had a "thenif" keyword, we would be able to shorten that code like this:
if my_condition thenif my_other_condition then
The new version of the code makes it harder to answer important questions at a glance, such as:
- Which branches might be taken if
my_condition is true?
- What happens if
my_other_condition is evaluated and comes out as false?
- Which occurrence of
else corresponds to which occurrence of