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Re: SRFI 105: Curly-infix-expressions

This page is part of the web mail archives of SRFI 105 from before July 7th, 2015. The new archives for SRFI 105 contain all messages, not just those from before July 7th, 2015.

John Cowan:
> Draft 7 isn't yet published, but soon will be.  Draft 6 is the current
> published draft.

Whups, of course I meant draft 6.

> R6RS says that `equal?` "returns #t if and only if the (possibly
> infinite) unfoldings of its arguments into regular trees are equal as
> ordered trees."  I sort of know what that means, but it isn't the kind
> of thing I'd like to put into a standard.  We have a window in the next
> few days to get better language into draft 7, if you have any on tap.

I have several concerns about cycle-detection in R7RS, which I think is otherwise shaping up pretty nicely.  Trying to deal with equal? led me to realize some weirdnesses with equal?, write, and display.  I'll see what I can do quickly.

> It's not obvious to me why you want to bother with this.  No Scheme I
> know of behaves well in the presence of cycles *in code*, and indeed
> R7RS prohibits them explicitly.  I know that curly-expressions can be
> used outside of code, but I expect (and I think you expect too) that
> code will be their dominant use.

True.  But since it will be used to read data, my concern was protecting the reader.

> I think it's a win to support `equal?`, with or without cycle detection.

Okay, fair enough.

> > > R6RS #!-switch is the closest I can think of that can make a source
> > > file self-descriptive about the lexical syntax it uses.  Did you
> > > think about using something like #!c-expr ?
> >
> > A switch might work, though I think the #! syntax for switches is
> > absurd; the #! sequence is ALREADY reserved by most Unix/Linux
> > systems, and they will not change it for Scheme :-(.
> Only as the first two characters of a file.  Putting a blank line before
> a Scheme-specific switch will defeat the Unix interpretation

Fair enough.  But will *Scheme* ignore it properly when it IS the first character of a file? Guile, clisp, and many other Lisps will ignore #!...!# so that you can create Scripts in Scheme / Common Lisp / whatever.  That is really useful functionality, and one that I hate to interfere with.

> (which, by
> the way, is not part of Posix, though quite pervasive in everything but
> Windows).

Yes, I know.  But the POSIX authors' rejection of #! has not exactly prevented its implementation :-).  I use #! all the time.  There's no reason Schemes can't support it, it makes scripting far more pleasurable.

> I think the SRFI should prescribe the switch (say, #!srfi-105),
> since there is no reason why it should be different on different
> implementations.

Okay, adding a switch seems to be strongly advocated, and I'm certainly amenable.  Any preference on the switch name?:
...something else...

--- David A. Wheeler