RepLib

Abstract

Some type class instances can be automatically derived from the structure of types. As a result, the Haskell language includes the "deriving" mechanism to automatic generates such instances for a small number of built-in type classes. In this paper, we present RepLib, a GHC library that enables a similar mechanism for arbitrary type classes. Users of RepLib can define the relationship between the structure of a datatype and the associated instance declaration by a normal Haskell functions that pattern-matches a representation type. Furthermore, operations defined in this manner are extensible-instances for specific types not defined by type structure may also be incorporated. Finally, this library also supports the definition of operations defined by parameterized types.In Section 7, we compare the capabilities of this proposal to existing work. For example, there are a number of ways to generically represent the structure of datatypes, and, more broadly, there are a number of ways to define type-indexed operations that do not rely on representation types. However, our view is that the success of any proposal relies on ease of adoption. Therefore, we have worked hard to identify a small set of mechanisms, implementable within the language of an existing Haskell compiler, that are, in our subjective view, useful for common situations and provide a programming model familiar to functional programmers.An initial release of RepLib is available for download 1 and is compilable with the Glasgow Haskell Compiler (GHC), version 6.4. This library is not portable. It requires many of the advanced features of GHC that are not found in Haskell 98: Higher-rank polymorphism [29], lexically-scoped type variables [31], Generalized Algebraic Datatypes (GADTs) [28], and undecidable instance declarations. Furthermore, Template Haskell [30] automates the definition of representations for new datatypes. However, all of these extensions are useful in their own respect.

Generic Programming in OCaml

“…The list of constructor view is similar to the underlying view of the RepLib Haskell library [38]. In a nutshell, the view sees all types as variants.…”

“…The open design of the library enables the user to define his own views. In addition to the low level view, we have included the sum-of-products view underlying the LIGD library [8] and Instant-Generics [7,27], the spine view underlying the SYB library [18] and the list-of-constructors view underlying RepLib [38].…”

“…LIGD [8] and Replib [38] use a GADT for type representation in the same way as we did with a ty, however their representation is closed and is fused with the view. Note that open variants are not natively supported in Haskell.…”

Datatype generic programming in F#

“…Over the last decade, datatype generic programming has emerged as a powerful mechanism for exploiting type structure to define families of functions. In Haskell alone, there are numerous tools and libraries for datatype generic programming, including PolyP (Jansson and Jeuring 1997), Generic Haskell (Hinze and Jeuring 2003), Scrap your boilerplate (Lämmel and Peyton Jones 2003), RepLib (Weirich 2006), Uniplate (Mitchell and Runciman 2007), Regular (Noort et al 2008), Multi-Rec (Yakushev et al 2009), Instant Generics (Magalhães and Jeuring 2011) and many others.…”

Comparing libraries for generic programming in haskell

“…• RepLib: a library for derivable type classes [Weirich, 2006] • Smash your boilerplate [Kiselyov, 2006] • Uniplate [Mitchell and Runciman, 2007b] Note that this list does not contain generic programming language extensions such as PolyP [Jansson and Jeuring, 1997], Generic Haskell ], or Template Haskell [Lynagh, 2003], and no pre-processing approaches to generic programming such as DrIFT [Winstanley and Meacham, 2006], and Data.Derive. We strictly limit ourselves to library approaches, which, however, might be based on particular compiler extensions.…”