Type-checking (bottom-up)

Expression checking is primarily done bottom-up. That is, the most deeply nested expressions are evaluated first, and their types are propagated upward in order to determine correctness.

(string-width (concat "a" "b"))

In this example, the first expressions evaluated are "a" and "b" , which each return literal types corresponding to their exact values.
The next expression evaluated is (concat "a" "b"). The arglist type &("a" "b") is matched against the function type &List<String> -> String, determining that the expression returns String.
Finally, the top-level expression is evaluated. The arglist type &(String) is matched against the function type &(String) -> Integer, determining that the expression returns Integer.