Macros and Scheme libraries

Macros are hygienic syntax-rules transformers, defined with define-syntax at the top level or in a local body. Surface forms such as let*, cond, case and the variadic list are macros in (snail-core base), and they expand away before checking, so the checker sees only the core.

A library file may hold a generate-library recipe instead of a define-library: the compiler builds and runs that program first, and compiles the library syntax it returns, which the program writes as bytes the compiler decodes rather than as text.

Scheme is a library too. (scheme base) gives R7RS names (cons, car, vector, char=?, …) over the typed language, and (scheme compiler) translates R7RS programs into Snail-Core libraries, run by a generate-library recipe, with (scheme runtime) for the standard procedures. Its character procedures are Unicode’s, from (snail-core text): char-upcase maps the full repertoire, char-numeric? and digit-value hold for any script’s decimal digits, and string-ci=? compares case-folded text, so "Straße" and "STRASSE" are equal. The Scheme on Snail-Core tutorial runs a Scheme program.

That compatibility rests on Snail-Core’s core being built so that erasing its typed forms leaves an R7RS program — which is also how Chibi Scheme runs the compiler’s own source. Bootstrap and self-hosting sets out the erasure property and the stages.

Full Scheme. (scheme compiler) is a Snail-Core library: a reader plus a translator from R7RS source to the syntax of a Snail-Core library. Every Scheme value is a Datum, the type of Scheme data in the dynamic libraries (datum, number, read, write), so matching on one enum replaces run-time type tests. Scheme code calls no typed Snail-Core code yet. Proposed: wherever generated code meets typed Snail-Core code, the translator inserts (type-convert e T), liberally and without proving it necessary:

• Upcast. Converting a typed value to Any is checked statically and injects the value with its TypeId.
• Downcast. Converting an Any to a specific type checks the TypeId at run time and traps on a mismatch; (type-test e T) asks first, without trapping.
• Redundancy. The compiler removes conversions it proves redundant, such as a downcast of a value just upcast from the same type.

On Chibi, type-convert erases to its expression. A generate-library recipe runs the translator:

(generate-library (legacy program)
                  (import
                   (snail-core base)
                   (snail-core io)
                   (scheme compiler))
                  (scheme->snail-core-library (list "legacy" "program") (read-file "program.scm")))