Typeview: A Tool for Understanding Type Errors

In modern statically typed functional languages, type inference is used to determine the type of each function automatically. Whenever this fails, the compiler emits an error message that is often very complex. Sometimes the expression mentioned in the type error message is not the one that is wrong. We therefore implement an interactive tool that allows programmers to browse through the source code of their program and query the types of each expression. If a variable cannot be typed, we would like to present a set of possible types from which the user can decide which is wrong. This should help finding the origin of type errors without detailed knowledge of type inference on the user side.