Implementing R7RS on R6RS Scheme requires resolving several incompatibilities between the two standards. This includes handling differences in define-library vs library forms, lexical notations like #u8() vs #vu8(), and properly resolving include paths when transforming R7RS libraries. Making both R6RS and R7RS libraries and notations work together allows interoperability and continued use of existing R6RS libraries in the future as R7RS adoption increases.

1. Implementing R7RS on R6RS Scheme
Takashi Kato
@tk_riple
AAutuotro:r : 1 91.91.11.11.414

2. AAutuotro:r : 1 91.91.11.11.414
Introduction
It's been a year since R7RS was standardised.
Numbers of libraries must be created by now.

3. Keyword Github Google Code
R6RS 59 18
R7RS 12 8
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Repositories
Keyword Github Google Code
R6RS 59 18
R7RS 12 8
*Searched in August 2014
*The numbers are not accurate
*Searched in August 2014
*The numbers are not accurate

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We can expect...
The number would grow in the near future
R6RS libraries are also useful
It would be nice to be able to use both libraries

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So we want to...
● Use R6RS libraries in R7RS libraries
● Use procedural macros in R7RS libraries
● Use R7RS libraries in R6RS libraries
● Use syntax-case in R7RS libraries
… so on
IImmpplleemmeenntt RR77RRSS oonn RR66RRSS!!

6. define-library vs library
export phasing
include
include-ci
cond-expand
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What needs to be done...
RReessoollvviinngg i innccoommppaattiibbiilliittiieess
define-library vs library
export phasing
include
include-ci
cond-expand
#u8() vs #vu8() #u8() vs #vu8()
|symbol escape| vs symbolx20;escape |symbol escape| vs symbolx20;escape

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define-library vs library
R7RS
● define-library
● Very flexible
– import, export, begin,
cond-expand and include
related clauses.
– These can be appear in
any order and any number
of times.
● No phasing
– R7RS has only syntax-rules
R6RS
● library
● Rather fixed form
– export and import with
respective order
● Phasing

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Lexical notations
R7RS
#u8( 1 2 3)
| f oo bar |
| f oo x20; bar |
. f oo
@f oo
f oo x20; bar
R6RS
#vu8( 1 2 3)
f oo x20; bar
| f oo bar |
. f oo
@f oo

9. Handling lexical notations...
● needs to be done by readers
– Basically no other way :)
● is easy but how should implementations write a
datum in proper notations?
– Using #! directives, like #! r 6r s?
– Oops, R7RS doesn't have #! r 7r s
● So using it would be implementation dependent
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10. Handling define-library forms...
● is the problem
– R7RS has extra keywords; include, include-ci, etc.
– But it doesn't have phasing keyword
● can be done in 2 ways;
– Expander should expand it to l i br ar y form
– Compiler should handle it
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11. Transforming R7RS library form...
● May require resolving proper phase
– Only for implementations adopted explicit phasing
● Requires tracking include or include-ci
– It is important to know where the source location is
– What if a macro contains include expression?
– What if include symbol is shadowed?
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Recall: Phasing
( l i br ar y ( f oo)
( expor t f oo)
( i mpor t ( f or ( r nr s) r un expand) )
( def i ne- synt ax f oo
( l ambda ( x)
( def i ne- synt ax name
( synt ax- rul es ( )
( ( _ k)
( dat um- >synt ax k ( s t ri ng- >symbol "bar" ) ) ) ) )
( synt ax- case x ( )
( ( k)
( wi t h- synt ax ( ( def ( name #' k) ) )
#' ( def i ne def ' bar ) ) ) ) ) ) )

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Resolving phasing
(foo) in R7RS
( def i ne- l i br ar y ( f oo)
( i mpor t …)
( expor t …)
( begi n
( def i ne- synt ax f oo
…) )
)
After the transformation
( l i br ar y ( f oo)
( expor t …)
( i mpor t ( f or …) …)
( def i ne- synt ax f oo
…)
)

14. How should this be resolved?
File hierarchy
/
+ foo.sld
+ impl/
+ bar.scm
+ buzz.scm
+ buzz.scm
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; ; f oo. sl d
( def i ne- l i br ar y ( f oo)
( i mpor t ( scheme base) )
( expor t bar )
( i ncl ude " i mpl / bar . scm" ) )
; ; i mpl / bar . scm
( i ncl ude " buzz. scm" )
; ; i mpl / buzz. scm
( def i ne bar ' bar )
; ; buzz. scm
( def i ne bar ' boo)

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Well...
R7RS doesn't specify how to resolve the file paths...
However,
It would be nice if it's resolved as C preprocessor does.
So symbol bar should be bound to f oo.

16. How should this be resolved?
; ; f i l e- a. scm
( def i ne- synt ax i ncl ude- i t
( synt ax- r ul es ( )
( ( _ f i l e)
( i ncl ude f i l e) ) ) )
; ; f i l e- b. scm
( i ncl ude- i t " buzz. scm" )
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From where 'buzz.scm'
should be resolved?
From 'file-b.scm' should
make more sense to users.

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To do that...
● Expander should know where include expressions
are evaluated.
● Plus, result of macro expansion
Thus expander needs to do not only transforming
define-libray to library but also expand macros...

18. Implementing on Sagittarius
● define-library as the built-in syntax
– Avoid implementing macro expander twice
– The compiler can use the compile time environment
● include and include-ci are resolved 2 ways
– As a library keyword, it's simply sliced
● Paths are resolved from where the library is located
– As a syntax, compiler considers bindings
● Paths are resolved from where the current file is.
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19. Handling lexical notations (2)
● The reader can read both R6RS and R7RS notations
– Also providing strict modes
● Writing R7RS bytevectors requires #!r7rs directive
– The default mode writes in R6RS style
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Interoperation
; ; Expor t i ng pr ocedur al macr o f r om R6RS l i br ar y
( l i br ar y ( ai f )
( expor t ai f )
( i mpor t ( r nr s) )
( def i ne- synt ax ai f
( l ambda ( x)
( synt ax- case x ( )
( ( ai f c t )
#' ( ai f c t ( i f #f #t ) ) )
( ( k c t e)
( wi t h- synt ax ( ( i t ( dat um- >synt ax #' k ' i t ) ) )
#' ( l et ( ( i t c) )
( i f i t t e) ) ) ) ) ) ) )

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Interoperation
; ; usi ng pr ocedur al macr o i n R7RS l i br ar y
( def i ne- l i br ar y ( f oo)
( i mpor t ( scheme base) ( ai f ) )
( expor t a)
( begi n
( def i ne a
( l et ( ( l i s ' ( ( a . 0) ( b . 1) ( c . 2) ) ) )
( ai f ( assq ' a l i s)
( cdr i t ) ) ) ) ) )
; ; bel ow can be bot h R6RS/ R7RS scr i pt
( i mpor t ( f oo) )
a

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Conclusion
● Making R7RS Scheme system on top of R6RS is not
an easy task
– Not so difficult either
● Availability of both R6RS and R7RS can be a big
advantage and good for future Scheme users

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Questions?