The Glasgow Haskell Compiler is a state-of-the-art, open source, compiler and
interactive environment for the functional language Haskell.  Highlights:

 * Supports the entire Haskell 2010 language plus a wide variety of
   extensions.
 * Has particularly good support for concurrency and parallelism, including
   support for Software Transactional Memory (STM).
 * Generates fast code, particularly for concurrent programs.
 * Works on several platforms including FreeBSD, Windows, Mac, Linux, most
   varieties of Unix, and several different processor architectures.
 * Has extensive optimisation capabilities, including inter-module optimisation.
 * Compiles Haskell code either directly to native code or using LLVM as a
   back-end.  It can also generate C code as an intermediate target for porting
   to new platforms.  The interactive environment compiles Haskell to bytecode,
   and supports execution of mixed bytecode/compiled programs.
 * Profiling is supported, both by time/allocation and various kinds of heap
   profiling.
 * Comes with several libraries, and thousands more are available on Hackage.