Re: [Scheme-reports] ANN: first draft of R7RS small language available

[Andre van Tonder <andre@x>]

On Sat, 16 Apr 2011, Alex Shinn wrote:

> > - p57. How do we implement "=>" exported from (scheme base) module ?
> >  "=>" itself is not a syntax or a procedure (i think).
>
> As you say, there is no "=>" bound in (scheme base).
> So long as it's not bound in the usage context it will
> thus work with `cond'.
>
> If it's bound to something else, then it won't work in
> `cond', whether there it had any binding in (scheme base)
> or not.  If you want to import a module that binds "=>",
> you need to rename it or except it.
>
> The only thing binding it to something would change
> is require programmers to go through the effort to always
> specifically import it.

This discussion has been had in R6RS and based on the discussion there the 
decision was made to make it and other literals bound in the base.  There were 
many reasons for it, among others:

- There is a very strong argument for requiring these literals to be bound. If
  they refer to bindings, they can be exported, imported, prefixed, renamed, or
  excluded. These are useful operations, and I think it is important for users
  to be able to rely on them to work portably.

- Consider, for example, using syntax
  case together with a user-written matcher library, both of which use the
  identifiers ... and _ with separate meanings (here prefixing or renaming would
  be useful).   If we cannot rename one of these sets, eiher the SYNTAX-CASE
  or the MATCH code-walker is going ot
  become confused if there are nested user matcher expressions.

- A teacher may want to make an R5RS module that excludes use of post-R6RS
  literals such as _ in SYNTAX-RULES and instead treats _ as an ordinary
  variable.  Can only be done if _ is bound.

- We cannot easily create a teaching language where => clauses are not allowed
  in COND unless => is bound.

- An unbound ELSE is anglo-chauvinism.  Most of the world is not anglo.  A
  french elementary school teacher may want to translate ELSE for her
  students.  Can only be done if bound.

- If literals are not bound, the teacher can translate QUOTE for her students
  but cannot translate UNQUOTE.

- The semantics of FREE-IDENTIFIER=? which is used to compare literals - should
  unbound identitifiers in different modules be free-identifier=? if they have
  the same symbol?  This is not obvious to me, and as an implemento I know
  from experience that it can complicate a macro explansion system to insist
  upon it

- Typically, such literals mcan be bound to a macro expanding to a syntax error,
  which is extremely useful for error checking.

- A lot of these literals are common enough names that users are likely to
  stop on them by accident.  Fi they are bound, the expander will thorw an
  redefinition error.  If they are unbound, however, code will just mysteriously
  stop working.

Andre

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