Constraint-free type error slicing

Abstract

Type error messages for ML-based languages tend to suffer from imprecise error locations – the type checker reports only one of many possible locations of an error. The notion of a type error slice corrects this by reporting all program locations that contribute to a given error (and no more). Previous work on producing type error slices required the use of a constraint-based type checker implementation. For most existing systems this would require substantial changes to well-tested and subtle pieces of code. In this work we show how to produce useful type error slices with an unmodified type checker. Other tools, such as automatic correction systems, can be layered on top of our system. We have implemented this technique on top of the Glasgow Haskell Compiler (GHC) and report our experiences.