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Re: Fix reader options for R6RS `get-datum'

From: Mark H Weaver
Subject: Re: Fix reader options for R6RS `get-datum'
Date: Sun, 16 Dec 2012 17:12:22 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.2 (gnu/linux)

Andreas Rottmann <address@hidden> writes:

> Mark H Weaver <address@hidden> writes:
>> Section 8.3 defines 'read' as follows:
>>   Reads an external representation from textual-input-port and returns
>>   the datum it represents. The read procedure operates in the same way
>>   as get-datum, see section 8.2.9.
>> I believe this last sentence clearly confirms my belief that 'read' and
>> 'get-datum' should recognize the same syntax.
> Well yes, R6RS `read' and R6RS `get-datum' need to understand the same
> syntax, but I thought you were talking about Guile `read' and R6RS
> `get-datum'.

Ah, so you want R6RS 'read' to be different than Guile 'read'.
I think this would be a mistake.

I'd like to allow coherent systems to be built from a mixture of R6RS
code, R7RS code, native Guile code, etc.  With this in mind, I think it
would be terribly confusing for users (and not particularly sensible)
for the notation recognized by 'read' to depend upon whether the code
that happens to call 'read' is in an R6RS library or a Guile module.

For example, the code that calls 'read' when compiling source files
happens to be in a Guile module.  What does that have to do with the
language being read?  Nothing.

> Yup, R6RS `read' needs to be implemented in terms of `get-datum', not
> only because of reader options, but also because of the required
> exception behavior.  This is how it's done already -- see
> modules/rnrs/io/simple.scm.

I thought we agreed on IRC that this is an unworkable approach to
supporting R6RS exceptions in Guile.  That path leads to a future where
there are two variants of every primitive procedure that might throw
exceptions.  It also means duplicating every VM instruction that might
throw exceptions.

Those facts alone would be bad enough, but it gets worse.  In a program
composed of a mixture of R6RS and native Guile code, an R6RS exception
handler should be able to properly catch an error that happened within
native Guile code, and vice versa.  That won't work with this approach
of throwing R6RS-style exceptions from within R6RS primitives and
Guile-style exceptions within Guile primitives.

IMO, to create a coherent system that allows mixing of code, we need a
single unified exception system that is sufficiently fine-grained (and
provides enough information) to satisfy the needs of both R6RS exception
handlers and legacy Guile exception handlers.

At any given time, there might be exception handlers installed by both
Guile 'catch' and R6RS 'guard'.  The code that throws an exception has
no way of knowing which kind of exception handler will catch it.
Therefore, the conversion to native R6RS conditions needs to happen
within the exception handler.

Does that make sense?  I thought we discussed this on IRC and agreed on
this general approach.

>> On the flip side, if someone has enabled SRFI-105 curly-infix
>> expressions, or any other reader extension that does not conflict with
>> standard R6RS notation, then both 'get-datum' and 'read' should honor
>> that setting.
>> Does that make sense?
> It does, and I think this is also what my patch implements, if I
> understood both the code and your words correctly :-).

To make this more concrete, let's consider two of the reader options
that you'd apparently like to override within R6RS code:

*** Case insensitivity (you would force case-sensitive mode in R6RS):

R6RS appendix B specifies the following optional reader directives:


and Guile 2.0.7 now supports this.  Your patch would break this when
'read' is used within R6RS code.  Furthermore, it would break in a
strange way: #!fold-case or #!no-fold-case would take affect for the
immediately following datum (or the containing datum if the directive is
found within a list), but then the reader would revert to case-sensitive
mode for subsequent datums.

*** Keyword style (you would disallow this option in R6RS):

While it is true that ':' is one of the "extended alphabetic characters"
allowed in identifiers (and therefore the standard requires that :foo be
read as a normal symbol), this has _always_ been the case in every
Scheme standard since at least the R2RS.  Nonetheless, some users want a
more convenient syntax for keywords, hence we have this reader option.

It is off by default, but some users prefer to have it on.  I don't see
why this setting should be ignored if the code that calls 'read' happens
to be in an R6RS library.

Furthermore, I intend to add another reader directive to set the keyword
option.  If you override this option, it will break in the same manner
as for #!fold-case as described above.

I have more to say about this issue, but this is enough for one email :)



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