Take ideas from Racket

[jariji]

Racket has contracts (guide, reference). There are a lot of research papers on it, especially

• "Contracts for higher-order functions"
• "Correct blame for contracts"

[Keno]

I don't really know much about Racket, but it looks like this facility is a semantic one, which I'm very explicitly trying not to do. That said, if there's particular features in this system that people think are nice, I'd like to hear about it.

[jariji]

Some ideas that come to mind are

• Blame, especially in higher-order functions. If map(f, x) goes wrong, whose fault is it: the caller for misusing one of those functions, or the developer of f, or map? It depends. With a first-class concept of "blame", the user is informed where the problem is in each case. See example, and papers linked above.

• Contract combinators, like the or combinator that takes two contracts and makes a contract that is satisfied if either of its components is.