Type error diagnosis for OutsideIn(X) in Helium

Abstract

Rhodium is a modification to the OutsideIn(X) type inference framework, the type inferencer used in the GHC compiler. Rhodium allows for the usage of heuristics for advanced type error diagnosis. Rhodium is successfully implemented in the Haskell compiler Helium, which is known for its high quality type error messages. Using Rhodium, we extend the language with Generalized algebraic datatypes (GADTs). Rhodium has also support for type families and multi-parameter type classes, while preserving the high quality of the type error messages. We will discuss how the type inferencer uses type graphs and heuristics to generate these error messages.