CLU
The CLU programming language is a general-purpose programming language known for its strong support of data abstraction and modular programming. It is often used for teaching and research in programming language design.
Hello, world code example:
start_up = proc ()
po: stream := stream$primary_output()
stream$putl(po, "Hello world!")
end start_up
Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.