regex-indef is a minimal library for parsing regular expressions. It is an indefinite library. It defines two things:

• A module named Regex (which contains an import of Str)
• A module signature named Str; we use a module signature here, rather than a module, to defer specifying exactly what type of strings the Regex module parses.

Each of the following packages provides an implementation of the Str module signature:

• str-string defines strings as String.
• str-bytestring defines strings as ByteString.

Then there are two empty packages, regex-string and regex-bytestring. Each of these packages combines regex-indef with a Sig implementation to produce a definite regular expression library.

• regex-string depends on regex-indef and str-string, and it reexports the Regex module as Regex.String.
• regex-bytestring depends on regex-indef and str-bytestring, and it reexports the Regex module as Regex.ByteString.

Summary of terminology

• Module - An .hs file.
• Module signature - An .hsig file.
• Package - A directory containing a .cabal file.
• Library - A library stanza within a .cabal file.
• Indefinite library - A library that has module signatures without implementations. (A library with a "hole" in it.)
• Definite library - A library for which all module signatures (if any) have implementations. (A library that is not missing anything.)
• Reexporting a module - To include a module in a library's API under a different name than the name with which the module was originally defined.

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See also: Edward Z. Yang's blog post Try Backpack: Cabal packages.