# yoko: generic programming with disbanded constructors

[ bsd3, generics, library, reflection ] [ Propose Tags ]

yoko views a nominal datatype as a band of constructors, each a nominal type in its own right. Such datatypes can be disbanded via the disband function into an anonymous sum of nominal constructors, and vice versa via the band function. This library uses extensive type-level programming to enrich its instant-generics foundation with capabilities derived from the constructor-centric perspective.

For example, consider the following nominal datatype.

data Beatles = John ... | Paul ... | George ... | Ringo ...


This type can of course be understood as a sum of the individual constructor types.

data John = John ...
data Paul = Paul ...
data George = George ...
data Ringo = Ringo ...


yoko's conceptual foundations start there. In particular, this allows a constructor, say John, to be used independently of its original range type and sibling constructors.

As a generic programming library, yoko extends instant-generics with support for constructor-centric generic programming. The Examples/LL.hs file distributed with the yoko source demonstrates defining a lambda-lifting conversion between the two types Inner, which has lambdas, and Prog, which has top-level function declarations instead.

data Inner = Lam Type Inner | Var Int | App Inner Inner

data Term = Var Int | App Term Term | DVar Int
data Prog = Prog ([Type], Type, Term) Term


These types are defined in separate modules, since they have constructors with the same name. Indeed, the fact that they having matching constructors named App is crucial for yoko's automatic conversion from Inner's App to Term's App. As written, the generic lambda-lifter would continue to work for any new Inner constructors (e.g. syntax for tuples or mutable references) as long as constructors with the same names and analogous fields were added to Term and the semantics of those constructors doesn't involve binding. This default behavior of the lambda-lifter is specified in about ten lines of user code.

Existing generic libraries don't use constructor names to the degree that yoko does, and so cannot accomodate generic conversions nearly as well.

See the wiki at http://code.google.com/p/yoko for more documentation.

See the documentation at http://code.google.com/p/yoko