Ministg is an interpreter for a high-level, small-step, operational semantics for the STG machine. The STG machine is the abstract machine at the core of GHC. The operational semantics used in Ministg is taken from the paper Making a fast curry: push/enter vs. eval/apply for higher-order languages by Simon Marlow and Simon Peyton Jones.
Ministg follows the rules in the paper very closely, to the point that each evaluation rule in the paper corresponds to an equation in the interpreter. This makes is easy to check that the interpreter follows the rules properly, and it also makes is easy to understand how new rules can be added, and what effect they might have.
One of the most useful features of Ministg is its ability to trace the steps of program execution. If tracing is enabled, Ministg will save each step of execution to a HTML file. Each such file contains the entire state of the STG machine at that point in the computation: namely the code, stack and heap. An example trace is shown here: . The files are linked together allowing you to step forwards and backwards through the execution.