Pico version of revised⁷ report on the algorithmic language scheme (This was originally called r7rs-tiny-spec)
This designed as a smaller version of the Scheme spec, suitable for use when the small version is still too big.
r7rs-pico (pr7rs) is designed to be small enough that a small group or a single person working on it part time can implement it in a few months.
In addition, the denotational semantics are considerably simpler than the semantics for r7rs-small which aids in understanding it.
For example, in pr7rs the type of an expression is:
U -> E
but in r7rs-small the type is:
U -> P -> K -> C
since the expression has to return a continuation and be able to modify a store. However in pr7rs an expression only has to convert an environment to a expressed value.
There are various other smaller versions of Scheme including:
- The Little Schemer has a smaller version used in the book.
- Bottom Scheme: https://github.com/johnwcowan/r7rs-work/blob/master/BottomScheme.md
- Lispy: https://norvig.com/lispy.html
- LISP in a Boot Sector: https://github.com/jart/sectorlisp
- PICOBIT: https://github.com/stamourv/picobit
In order to use the examples in The Little Schemer in Pico Scheme, the following definitions are needed:
(define eq? eqv?)
(define zero? (lambda (x) (= x 0)))
(define add1 (lambda (x) (+ x 1)))
(define sub1 (lambda (x) (- x 1)))
(define atom? (lambda (x) (or (symbol? x) (number? x))))There is an interpreter for pr7rs that mostly implements this specification: https://github.com/jrincayc/rust_pr7rs/