The Spineless Tagless G-machine is an abstract machine designed to support nonstrict higher-order functional languages. This presentation of the machine falls into three parts. Firstly, we give a general discussion of the design issues involved in implementing non-strict functional languages. Next, we present the STG language, an austere but recognisably-functional language, which as well as a denotational meaning has a well-defined operational semantics. The STG language is the "abstract machine code" for the Spineless Tagless G-machine. Lastly, we discuss the mapping of the STG language onto stock hardware. The success of an abstract machine model depends largely on how efficient this mapping can be made, though this topic is often relegated to a short section. Instead, we give a detailed discussion of the design issues and the choices we have made. Our principal target is the C language, treating the C compiler as a portable assembler. Version 2.5 of this paper (minus appendix) appe...

The garbage collector presented in this paper makes use of two well known compaction garbage collection algorithms with very different performance characteristics: Cheney's two-space copying collector and Jonker's sliding compaction collector. We propose a scheme which allows either collector to be used. The run-time memory requirements of the program being executed are used to determine the most appropriate collector. This enables us to achieve a fast collector for heap requirements less than half of the heap memory but allows the heap utilization to increase beyond this threshold. Using these ideas we develop a particularly attractive extension to Appel's generational collector. We also describe a particularly fast implementation of the garbage collector which avoids interpreting the structure and current state of closures by attaching specific code to heap objects. This code knows the structure and current state of the object and performs the appropriate actions without having to te...