How type errors were fixed and what students did?

Abstract

Providing better supports for debugging type errors has been an active research area in the last three decades. Numerous approaches from different perspectives have been developed. Most approaches work well under certain conditions only, for example, when type errors are caused by single leaves and when type annotations are correct. However, the research community is still unaware of which conditions hold in practice and what the real debugging situations look like. We address this problem with a study of 3 program data sets, which were written in different years, using different compilers, and were of diverse sizes. They include more than 55,000 programs, among which more than 2,700 are ill typed. We investigated all the ill-typed programs, and our results indicate that current error debugging support is far from sufficient in practice since only about 35% of all type errors were caused by single leaves. In addition, type annotations cannot always be trusted in error debuggers since about 30% of the time type errors were caused by wrong type annotations. Our study also provides many insights about the debugging behaviors of students in functional programming, which could be exploited for developing more effective error debuggers.