Formally Deriving an STG Machine

摘要

Starting from P. Sestoft semantics for lazy evaluation, we define a new semantics in which normal forms consist of variables pointing to lambdas or constructions. This is in accordance with the more recent changes in the Spineless Tagless G-machine (STG) machine, where constructions only appear in closures (lambdas only appeared in closures already in previous versions). We prove the equivalence between the new semantics and Sestoft's. Then, a sequence of STG machines are derived, formally proving the correctness of each derivation. The last machine consists of a few imperative instructions and its distance to a conventional language is minimal. The paper also discusses the differences between the final machine and the actual STG machine implemented in the Glasgow Haskell Compiler.