SISAL

SISAL is a functional programming language that supports data parallelism and high-level array operations. It is mainly used for scientific and numerical computing applications.

Hello, world code example:

define main

% Sisal doesn't yet have a string built-in.
% Let's define one as an array of characters.

type string = array[character];

function main(returns string)
  "Hello world!"
end function

Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.

• Wikipedia page