Modular implicits

Abstract

We present modular implicits, an extension to the OCaml language for ad-hoc polymorphism inspired by Scala implicits and modular type classes. Modular implicits are based on type-directed implicit module parameters, and elaborate straightforwardly into OCaml's first-class functors. Basing the design on OCaml's modules leads to a system that naturally supports many features from other languages with systematic ad-hoc overloading, including inheritance, instance constraints, constructor classes and associated types.

First-Class Subtypes

“…In OCaml the waters are muddied by side-effects and nontermination. contexts, which are passed as implicit arguments [20]. (Lifts through contravariant contexts are defined below in terms of the minimal interface.)…”

“…This corresponds to the Church encoding of the natural numbers [8], in which a natural number is anything that can produce a M.t from M.zero : M.t and M.suc : M.t → M.t. Here and elsewhere we're using the modular implicits extension to OCaml [20] -not for implicit instantiation of arguments, but because modular implicits support higher-kinded quantification with propagation of variance information. Other approaches to higher-kinded polymorphism could perhaps be used instead [22].…”

“…The code in this paper uses the proposed modular implicits extension to OCaml[20]. The modular implicits compiler can be installed as the OPAM switch 4.02.0+modular-implicits.…”

Generic Programming in OCaml

“…It is theoretically possible to infer the type witness since there is a one to one correspondence between the witnesses and types. The work on modular implicits [39] promises to offer this functionality.…”

“…The spine view is implicit much like in the Haskell implementation. The Haskell Typeable and Data type classes are directly translated as a modules, using the correspondence explained in [39].…”