I have heard of the "J operator" or "program point operator", when researching about ISWIM. I would like to know what it is. The Wikipedia article for it is very vague:
In computer science, Peter Landin's J operator is a programming construct that post-composes a lambda expression with the continuation to the current lambda-context. The resulting “function” is first-class and can be passed on to subsequent functions, where if applied it will return its result to the continuation of the function in which it was created.
What does "post-composing" mean? I checked and could only find a mathematical definition. I searched a lot elsewhere and have found very little. There are a few pages that mention it but none describe in detail what it actual is.
So, what is the J operator, what does it do and is it similar to call/cc?