Yap is an interactive PostScript previewer for developers who want to write and test PostScript code. Yap lets you enter, edit, and execute PostScript code on the fly and allows you to read and write text files so the code can be used elsewhere. Yap is intended for experimenting with short, hand-created segments of PostScript. It's not useful for previewing page-oriented documents, because it ignores all Encapsulated PostScript comments (such as %%BoundingBox and %%Page). For viewing page-oriented documents, use the Preview application located in /System/Applications..

The chapter also contains information about a related program, pft, which you can use if you need to communicate with the PostScript Window Server. pft is a command-line utility that runs in a Terminal window, so for general-purpose PostScript editing and viewing it's easier to use Yap.