www.cs.uu.nl Explicit implicit parameters

In almost all languages arguments to functions are to be given explicitly in the program text. There are however a few interesting exceptions to this rule. In Haskell functions take arguments which are either passed explicitly or implicitly. An instance of the latter is the class system in Haskell where dictionaries are passed as evidence for class predicates. However, the construction as well as the passing of these dictionaries is invisible to the programmer. Unfortunately the approach taken here is that the language is a bit autistic in the sense that the programmer cannot provide any help if the built-in proof mechanism fails. In this paper we propose, in the context of Haskell, a mechanism that allows the programmer to explicitly pass implicit parameters. This extension blends well with existing resolution mechanisms for determining which implicit parameters have to be passed, since it only overrides default the behavior of such mechanisms. We also describe how this extension can be implemented for Haskell. The implementation also gives us the additional bonus of partial type signatures, liberating the programmer from the obligation to specify either full signatures or not to specify a signature at all.