C/C++ pretty printing library

This library (or libraries, there being one library for C and one with C++ bindings) implements the interface described in Wadler's paper "A Prettier Printer".

Differences from the paper

The pretty-printing combinators do not reduce or simplify when called so that the base C API may exist without making any memory allocations. All reduction is done when pretty-printing, which is likely an acceptable tradeoff since one is likely to only pretty-print the structure once, or in some cases maybe a few times with varying settings and/or different outputs.

Not all higher-level functions shown in the paper are implemented, but the basic nil, text, sep (not explicitly mentioned in the paper, but often found in implementations as a soft line break), line, nest, append, and group are implemented, as well as some functions to make the C api more friendly.

C API

The C api is documented in prettyprint.h. It has a basic set of functions that make no memory allocations, and then equivalent functions that use malloc. The intent is that most users use the malloc-ing functions.

Memory Ownership

The provided pp_free function assumes nested documents are owned by the parent document, but other than that makes no assumptions about ownership. This is particularly important with regard to the text document: it does not assume the string is owned (and thus you may leak memory if they are not cleaned up properly). Use extensions to introduce memory ownership semantics.

Extensions

You may specify custom document types. These types must be at or above PP_DOC_EXTENSION_START. These documents may be allocated however you choose, and may contain any extra data. Read some more detail in the documentation for the evaluate_extension member of the pp_settings struct.

The gist of it is that you may specify an extension evaluator in the pretty-print settings, and this evaluator is used when extensions are encountered. It also gets a reference to the settings, so you may add extra settings used by extensions to a custom settings object (that has pp_settings somewhere in it). See the c example for inspiration. That example adds document types that

take memory ownership of a string (assuming pp_free_ext is used),
print the time when pretty-printed, and
can be filtered based on an added setting.

C++ API

Like the C api, prettyprint.h contains the C++ API. Though not fully documented, the functions/classes should be mostly self-explanatory. See the c++ example for an idea of how it works. The C++ API is a wrapper over the C API, so it behaves very similarly.