Scheme 48 is a programming language, a dialect of the language Scheme, an implementation using an interpreter which emits bytecode.^[2] It has a foreign function interface for calling functions from the language C^[3] and comes with a library for regular expressions (regex),^[4] and an interface for Portable Operating System Interface (POSIX).^[5] It is supported by the portable Scheme library SLIB, and is the basis for the Scheme shell Scsh.^[2] It has been used in academic research.^[6] It is free and open-source software released under a BSD license.

It is called "Scheme 48" because the first version was written in 48 hours in August 1986.^[7] The authors now^[when?] say it is intended to be understood in 48 hours.^{[citation needed]}

Implementation

Scheme 48 uses a virtual machine to interpret the bytecode, which is written in a restricted dialect of Scheme called PreScheme, which can be translated to C and compiled to a native binary. PreScheme, or Pre-Scheme, is a statically-typed dialect of Scheme with the efficiency and low-level machine access of C while retaining many of the desirable features of Scheme.

Pre-scheme was quite interesting. Kelsey published a paper on it, as well, I believe. It was Scheme in the sense that you could load it into a Scheme system and run the code. But it was restrictive – it required you to write in a fashion that allowed complete Hindley-Milner static type inference, and all higher-order procedures were beta-substituted away at compile time, meaning you could *straightforwardly* translate a prescheme program into "natural" C code with C-level effiency [sic]. That is, you could view prescheme as a really pleasant alternative to C for low-level code. And you could debug your prescheme programs in the interactive Scheme development environment of your choice, before flipping a switch and translating to C code, because prescheme was just a restricted Scheme. The Scheme 48 byte-code interpreter was written in prescheme. Prescheme sort of died – beyond the academic paper he wrote, Kelsey never quite had the time to document it and turn it into a standalone tool that other people could use (Ian Horswill's group at Northwestern is an exception to that claim – they have used prescheme

— Olin Shivers, "Olin Shivers: History of T"^[8]