Modern compilers optimize code during the compilation process. The goal of the optimizations is to obtain a program that is semantically equivalent, executes faster and uses less memory.

The Glasgow Haskell Compiler also uses a simplifier. This simplifier is not modular and difficult to extend. The HsOpt simplifier is a 'simplified' GHC simplifier. The HsOpt simplifier is modular and relative easy to extend. HsOpt is based on the paradigm of correctness preserving rewrite rules. The transformations can be applied separately.

HsOpt interacts with the Helium compiler, a compiler for a large subset of Haskell. Helium uses a naive pattern matcher compiler which results in straight forward intermediate Core code. HsOpt simplify most of the straight forward code. The result is Core code that could have been produced by an intelligent pattern match compiler.

The simplified program executes 10-15% faster and uses 0-15% less memory.