Enumerating Counter-Factual Type Error Messages with an Existing Type Checker (poster+demo)

The Hindley-Milner type system is a foundation for most statically typed

functional programming languages, such as ML, OCaml and Haskell. This type

system has many advantageous, but it does make type debugging hard: If a

program is not well-typed, it can be difficult for the programmer to locate the

cause of the type error, that is, to determine where to change the program how.

Many solutions to the problem have been proposed. Here we propose a new

solution with two distinctive advantages: It is easy to use for the functional

programmer, because it appears to be only a minor extension of the type error

messages they are already familiar with. It is easy to implement, because it does

not require the implementation of a new type checker, but instead reuses any

existing one as a subroutine (like [2]).