Eight years ago, functional dependencies, a concept from the theory of relational databases, were proposed as a mechanism for avoiding common problems with multiple parameter type classes in Haskell. In this context, functional dependencies give programmers a means to specify the semantics of a type class more precisely, and to obtain more accurate inferred types as a result. As time passed, however, several issues were uncovered---both in the design of a language to support functional dependencies, and in the ways that programmers use them---that led some to search for new, better alternatives.
This paper focusses on two related aspects of design for functional dependencies: (i) the design of language/type system extensions that implement them; and (ii) the design of programs that use them. Our goal is to clarify the issues of what functional dependencies are, how they should be used, and how the problems encountered with initial proposals and implementations can be addressed.