As of GHC version 6.8, we have adopted the following policy for numbering GHC versions:
Stable branches are numbered x.yy is even.
	  Releases on the stable branch x.yx.y.zz (>= 1) is the patchlevel number.
	  Patchlevels are bug-fix releases only, and never
	  change the programmer interface to any system-supplied code.
	  However, if you install a new patchlevel over an old one you
	  will need to recompile any code that was compiled against the
	  old libraries.
The value of __GLASGOW_HASKELL__
	  (see Section 4.12.3, “Options affecting the C pre-processor”) for a major release
	  x.y.zxyy (if
	  y is a single digit, then a leading zero
	  is added, so for example in version 6.8.2 of GHC we would have
	  __GLASGOW_HASKELL__==608).
We may make snapshot releases of the current stable branch available for download, and the latest sources are available from the git repositories.
Stable snapshot releases are named
	  x.y.z.YYYYMMDDYYYYMMDDx.y.z+16.8.1.20040225 would be a
	  snapshot of the 6.8 branch during the development
      of 6.8.2.
The value of __GLASGOW_HASKELL__
	  for a snapshot release is the integer
	  xyy.  You should never write any
	  conditional code which tests for this value, however: since
	  interfaces change on a day-to-day basis, and we don't have
	  finer granularity in the values of
	  __GLASGOW_HASKELL__, you should only
	  conditionally compile using predicates which test whether
	  __GLASGOW_HASKELL__ is equal to, later
	  than, or earlier than a given major release.
We may make snapshot releases of the HEAD available for download, and the latest sources are available from the git repositories.
Unstable snapshot releases are named
	  x.y.YYYYMMDDYYYYMMDD6.7.20040225 would be a
	  snapshot of the HEAD before the creation of the
      6.8 branch.
The value of __GLASGOW_HASKELL__
	  for a snapshot release is the integer
	  xyy.  You should never write any
	  conditional code which tests for this value, however: since
	  interfaces change on a day-to-day basis, and we don't have
	  finer granularity in the values of
	  __GLASGOW_HASKELL__, you should only
	  conditionally compile using predicates which test whether
	  __GLASGOW_HASKELL__ is equal to, later
	  than, or earlier than a given major release.
The version number of your copy of GHC can be found by
    invoking ghc with the
    ––version flag (see Section 4.6, “Help and verbosity options”).