[CM] Delimited strings in s7

[Elijah Stone]

On Mon, 5 Apr 2021, bil at ccrma.Stanford.EDU wrote:
> But I'm not sure what you want here. There are a lot of examples in 
> lint.scm (line 23201).

My point is, in a _general_ sense the current design of #readers makes it 
difficult to add new types of tokens, because of the 'read until next 
delimiter' behaviour.

Let's say, for example--not very practical, but you get the point--that I 
want an alternate syntax for character literals with #&x.  Unlike #\, #& 
should only ever accept a single character; so #&a is the same as #\a, but 
#&Newline is the same as #\N ewline (latter is two tokens). Under the 
current scheme, the reader function will get passed "&Newline" as its 
parameter, and need to manually return (values #\N 'ewline).  To make 
matters worse, a token can be split across the partially-read string and 
the input port; for instance, consider #&x#&(.  This should be the same as 
#\x #\(, but the reader will pass the '&' sharp reader "&x#&", and "(" 
will be left on the input port; and the '&' sharp reader somehow has to 
reconstruct the result.

My proposal is to rectify this in a backwards-compatible way by checking 
the arity of #-readers; if a #-reader takes 1 parameter, the behaviour is 
the same as it is currently, but if it takes 0 parameters, then the reader 
will not read any characters past the first '#'.

  -E