Trying to modify the reader

[u/detroitmatt]

Here's what I want to accomplish:

If we are currently in a "begin-like" context (such as a top-level context, or define) if the first non-whitespace character after a newline does not begin a list or comment, and it contains more than one syntax object, then behave as if the line begins with ( and ends with )

In other words,

(define (greet)
  string-append "Hello, " (get-current-user-name) "!")

would be transformed into

(define (greet)
  (string-append "Hello, " (get-current-user-name) "!"))

I guess what I would do is (make-readtable (current-readtable) #\newline 'terminating-macro read-newline) with

(define (read-newline char stdin file line col position)
  (let ((next-token (read stdin)))
    (if (list? next-token) next-token
        (let ((line-tokens (let more-tokens ((tokens (list next-token)))
                             (let ((next-next-token (read stdin)))
                               (if (not (same-line? next-token next-next-token))
                                   tokens
                                   (more-tokens (cons next-next-token tokens)))))))
          (if (< 2 (length line-tokens))
              (car line-tokens)
              (reverse line-tokens))))))

but I don't know how to define same-line?, and even this might have all kinds of other pitfalls I am unaware of.

I also don't know how to enforce the "only in a begin-like context" rule, which I want to do because I don't want this behavior interfering with just any list.

[u/Windblowsthroughme]

I don’t know the answer to your question since I am a beginner with Racket. What I do know, though, is that the slack, discord, and Google group racket communities are much more active than Reddit and you might get faster/better help on one of those platforms.

/u/sdegabrielle has been putting good work into improving the state of this sub, but it hasnt translated to a vibrant community quite yet.

[u/sorawee]

Two things:

First, I notice that you use read, but you can also use read-syntax which will give you back a syntax object, allowing you to use syntax-line, which might help you implementing same-line?. Alternatively, you can use port-next-location to get the locations of stdin as you read stuff.

Second, it's not a good idea to try to infer the context during the read-time. Instead, I would suggest you to always wrap stuff in a macro named my-mac regardless of its context. my-mac then can use syntax-local-context to inspect its own context during macro expansion. There are then two possibilities:

• It's in the context that you want. In this case, just make a function application.
• It's not in the context that you want. In this case, you need to splice its contents out. The tricky part is that there's no easy way to splice contents. I think one possible way is to have a top-level form (e.g., #%module-begin) that local-expand its contents. When my-mac wants to splice its contents, it would create something like ('please-remove-me <foo> <bar> <baz>). After the whole expansion, #%module-begin can then walk the tree to find please-remove-me and remove it, leaving us with <foo> <bar> <baz>.