Functional Pearl: Implicit Conï¬gurations â€”or, Type Classes Reï¬‚ect the Values of Types, by Oleg Kiselyov and Chung-chieh Shan:
The conï¬gurations problem is to propagate run-time preferences throughout a program, allowing multiple concurrent conï¬guration 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 conï¬gurations 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 conï¬guration 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 programâ€™s performance by transparently dispatching to specialized code at run-time. We can propagate any type of conï¬guration dataâ€”numbers, strings, IO actions, polymorphic functions, closures, and abstract data types. No previous approach to propagating conï¬gurations 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. Conï¬guration 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.