1.3. GHC version numbering policy
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## 1.3. GHC version numbering policy

As of GHC version 6.0, we have adopted the following policy for numbering GHC versions:

Stable Releases

  These are numbered ``x`.`y`.`z``, where `y` is even, and `z` is the patchlevel number (the trailing `.`z`` can be omitted if `z` is zero). 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 4.10.3, “Options affecting the C pre-processor”) for a major release ``x`.`y`.`z`` is the integer `xyy` (if `y` is a single digit, then a leading zero is added, so for example in version 6.2 of GHC, `__GLASGOW_HASKELL__==602`).

Snapshots/unstable releases

  We may make snapshot releases of the current development sources from time to time, and the current sources are always available via the CVS repository (see the GHC web site for details).

  Snapshot releases are named ``x`.`y`.YYYYMMDD` where `YYYYMMDD` is the date of the sources from which the snapshot was built. In theory, you can check out the exact same sources from the CVS repository using this date.

  If `y` is odd, then this is a snapshot of the CVS HEAD (the main development branch). If `y` is even, then it is a snapshot of the stable branch between patchlevel releases. For example, `6.3.20040225` would be a snapshot of the HEAD, but `6.2.20040225` would be a snapshot of the `6.2` 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 4.5, “Help and verbosity options”).

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