Origami is an un-SYB Haskell framework for transforming heterogenous data through folds. It consists of a coding pattern for transforming heterogenously typed data and Template Haskell macros to support the pattern. It operates in the same problem space as SYB, but generates the boilerplate for the user.
In the pattern, we focus on a specific set of mutually recursive datatypes, the fold family. The user specifies one or more root datatypes to be included and all datatypes recursively referenced by the root will be included. The user may specify datatypes to be treated as constant by the folds; these datatypes will be treated as atomic and not be further analysed.
For each constructor of each of those datatypes, the user may define a function to "replace" the constructor when folding. These functions are collected together into a single record. (Its declaration is generated for the user.) Collecting the functions together forces the typing of all the functions to remain consistent and facilitates code reuse.
For each datatype, a fold function is automatically generated that applies the fold record to values of that datatype. It decomposes the value into parts according to its constructor, recursively applies the fold to the parts, then puts them back together using the replacement constructor function from the record.
As with SYB, Origami lets the user think on a higher-level of abstraction about data transformation and relieves her from the need to write boilerplate.
Code examples to be added.