The Pre-Scheme Restoration (
from to on 22 Jun 2024 23:09

Pre-Scheme is an interesting Scheme dialect which is being ported to modern systems. The language, why its being ported, and the porting effort are described in the linked post.

As described in the Pre-Scheme paper, the language offers a unique combination of features:

  • Scheme syntax, with full support for macros, and a compatibility library to run Pre-Scheme code in a Scheme interpreter. The compiler is also implemented in Scheme, enabling both interactive development and compile-time evaluation.
  • A static type system based on Hindley/Milner type reconstruction, as used in the ML family of languages (eg. Standard ML, OCaml, Haskell). Pre-Scheme supports parametric polymorphism, and has nascent support for algebraic data types and pattern matching, which are recently gaining popularity in mainstream languages.
  • An optimizing compiler targeting C, allowing for efficient native code generation and portable low-level machine access. C remains the common interface language for operating system facilities, and compatibility at this level is essential for modern systems languages.

Due to the restrictions of static typing and the C runtime model, Pre-Scheme does not (currently) support many of Scheme’s high-level features, such as garbage collection, universal tail-call optimization, heap-allocated runtime closures, first-class continuations, runtime type checks, heterogenous lists, and the full numeric tower. Even with these limitations, Pre-Scheme enables a programming style that is familiar to Scheme programmers and more expressive than writing directly in C.

Ironically, Pre-Scheme’s original purpose was to implement the interpreter for another Scheme dialect, Scheme 48 (Wikipedia). But the lower-level Pre-Scheme is now getting its own attention, in part due to the popularity of Rust and Zig. Pre-Scheme has the portability and speed of C (or at least close to it), but like Rust and Haskell it also has a static type system with ADTs and parametric polymorphism; and being a LISP dialect, like most other dialects it has powerful meta-programming and a REPL.


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