Functional Pearl: Implicit Conﬁgurations —or, Type Classes Reﬂect the Values of Types, by Oleg Kiselyov and Chung-chieh Shan:
The conﬁgurations problem is to propagate run-time preferences throughout a program, allowing multiple concurrent conﬁguration sets to coexist safely under statically guaranteed separation. This problem is common in all software systems, but particularly acute in Haskell, where currently the most popular solution relies on unsafe operations and compiler pragmas.
We solve the conﬁgurations problem in Haskell using only stable and widely implemented language features like the type-class system. In our approach, a term expression can refer to run-time conﬁguration parameters as if they were compile-time constants in global scope. Besides supporting such intuitive term notation and statically guaranteeing separation, our solution also helps improve the program’s performance by transparently dispatching to specialized code at run-time. We can propagate any type of conﬁguration data—numbers, strings, IO actions, polymorphic functions, closures, and abstract data types. No previous approach to propagating conﬁgurations implicitly in any language provides the same static separation guarantees.
The enabling technique behind our solution is to propagate values via types, with the help of polymorphic recursion and higher-rank polymorphism. The technique essentially emulates local type-class instance declarations while preserving coherence. Conﬁguration parameters are propagated throughout the code implicitly as part of type inference rather than explicitly by the programmer. Our technique can be regarded as a portable, coherent, and intuitive alternative to implicit parameters. It motivates adding local instances to Haskell, with a restriction that salvages principal types.
This paper has been mentioned in threads quite a few times, but never had its own story. Of particular interest is the discussion of how to safely support local type class instances where Haskell only supports global instances.