base is:module

Extensible exceptions, except for multiple handlers.
Basic data types and classes.
Class of mutable vectors.
Basic definitions for the HUnit library. This module contains what you need to create assertions and test cases and combine them into test suites. This module also provides infrastructure for implementing test controllers (which are used to execute tests). See Test.HUnit.Text for a great example of how to implement a test controller.
Basis for IArray and MArray. Not intended for external consumption; use IArray or MArray instead.

WARNING

This module is considered internal. The Package Versioning Policy does not apply. The contents of this module may change in any way whatsoever and without any warning between minor versions of this package. Authors importing this module are expected to track development closely.
Utils for calculating general worst, bound, squeese and free, functions. as per: "A Generalized Algorithm for Graph-Coloring Register Allocation" Michael Smith, Normal Ramsey, Glenn Holloway. PLDI 2004 These general versions are not used in GHC proper because they are too slow. Instead, hand written optimised versions are provided for each architecture in MachRegs*.hs This code is here because we can test the architecture specific code against it.
Put common type definitions here to break recursive module dependencies.
Base LLVM Code Generation module Contains functions useful through out the code generator.
Types for the general graph colorer.
Definitions of Request and Response types along with functions for normalizing them. It is assumed to be an internal module; user code should, if possible, import Network.HTTP to access the functionality that this module provides. Additionally, the module exports internal functions for working with URLs, and for handling the processing of requests and responses coming back.
General parsers, functions and datatypes for all Shakespeare languages.

Convenience header for basic GObject-Introspection modules

See the documentation for each individual module for a description and usage help.
Reexports from Data.* and GHC.* modules of base package.
Base types and combinators.
Classes and instances for Regex matching. This module merely imports and re-exports the common part of the new api: Text.Regex.Base.RegexLike and Text.Regex.Base.Context. To see what result types the instances of RegexContext can produce, please read the Text.Regex.Base.Context haddock documentation. This does not provide any of the backends, just the common interface they all use. The modules which provide the backends and their cabal packages are: In fact, just importing one of the backends is adequate, you do not also need to import this module.

Example

The code
import Text.Regex.Base
import Text.Regex.Posix ((=~),(=~~)) -- or TDFA or PCRE or ...

main = do
print b
print c
print d
where
b :: Bool
b = ("abaca" =~ "(.)a")
c :: [MatchArray]
c = ("abaca" =~ "(.)a")
d :: Maybe (String,String,String,[String])
d = ("abaca" =~~ "(.)a")
will output
True
[array (0,1) [(0,(1,2)),(1,(1,1))],array (0,1) [(0,(3,2)),(1,(3,1))]]
Just ("a","ba","ca",["b"])
Base Functors for standard types not already expressed as a fixed point.