Writing a parser combinator library does not require a powerful type system, especially not a custom one for a specific task (the students who were doing the sc

Writing a parser combinator library does not require a powerful type system, especially not a custom one for a specific task (the students who were doing the scanner/parser task were limited to an environment where they had no access to third-party libraries, not that any existed for the language they were given). For a simple parser combinator implementation from scratch you require nothing more than basic types and the type of whatever AST you're generating. Parametric polymorphism is useful when you want to write a reusable parser combinator library, something that wasn't necessary, but even then you don't really need more than that (see other existing parser combinator libraries). Higher order functions can make your life easier, but note that the students had access to a Java-like language (of which there was both a statically and a dynamically typed version) with Ruby-style closures, so that wasn't an obstacle.