I am trying to write a custom current-read-interaction for a language
with non-s-expression syntax, and I’m not sure I completely understand
the protocol for how to detect the end of input across both the command
line and DrRacket. I first wrote an implementation for DrRacket, which
appears to continually call current-read-interaction until it returns
#<eof>, at which point it presents the user a new prompt. Therefore, I
simply decided to check char-available? on the provided port and return
#<eof> whenever the port returned #f. This seems to work fine in
DrRacket.
On the other hand, I found it does not work at all on the command line.
Returning #<eof> on the command line just seems to kill the entire REPL.
Furthermore, unlike DrRacket, the CLI does not seem to produce an EOF at
the end of each expression, just a newline, re-using the same port each
time. Checking for a newline in DrRacket would not work at all, though,
because DrRacket permits entering multiline expressions in its REPL.
This would lead me to believe that there are two completely different
protocols for how the two different REPLs work:
1. In DrRacket, read all the contents in the port provided to
current-read-interaction. Check the port against char-available?
each call, and return #<eof> when it returns #f.
2. At the command line, read all contents up until a newline. Check
the port against peek-char each call, and return #<eof> when it
returns #<eof> to support exiting the REPL by sending ^D.
These seem totally different, frustratingly so, since it seems like it
would not be too hard for the CLI to use the same behavior as DrRacket.
My guess is that I’m overcomplicating things at least a little, but I
wasn’t able to locate anything about what exactly is expected of the
function provided to current-read-interaction. Is there a way to
implement it that can gracefully handle both scenarios? If not, what is
the proper way to check if code is running in the CLI REPL or the
DrRacket REPL and adjust behavior accordingly?
Alexis
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