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| author | Ryan Scott <ryan.gl.scott@gmail.com> | 2016-09-30 15:52:02 -0400 |
|---|---|---|
| committer | Ryan Scott <ryan.gl.scott@gmail.com> | 2016-09-30 15:52:02 -0400 |
| commit | b3d55e20d20344bfc09f4ca4a554a819c4ecbfa8 (patch) | |
| tree | 82835fbae36500ddd2f930d251314c3a409d41cb | |
| parent | 28a00eafd7abc474e6e5c3d92d3c0ec561906c0b (diff) | |
| download | haskell-b3d55e20d20344bfc09f4ca4a554a819c4ecbfa8.tar.gz | |
Document Safe Haskell restrictions on Generic instances
Commit 578fbeca31dd3d755e24e910c3a7327f92bc4ee3 disallowed hand-written
`Generic` instances in Safe Haskell, but this wasn't documented anywhere.
| -rw-r--r-- | docs/users_guide/safe_haskell.rst | 9 |
1 files changed, 9 insertions, 0 deletions
diff --git a/docs/users_guide/safe_haskell.rst b/docs/users_guide/safe_haskell.rst index 3914820ebe..6ce8b8fc26 100644 --- a/docs/users_guide/safe_haskell.rst +++ b/docs/users_guide/safe_haskell.rst @@ -278,6 +278,15 @@ Furthermore, we restrict the following features: between types. Derived instances (through the :ghc-flag:`-XDeriveDataTypeable` extension) are still allowed. +- ``GHC.Generics`` — Hand crafted instances of the ``Generic`` type class are + not allowed in Safe Haskell. Such instances aren't strictly unsafe, but + there is an important invariant that a ``Generic`` instance should adhere to + the structure of the data type for which the instance is defined, and + allowing manually implemented ``Generic`` instances would break that + invariant. Derived instances (through the :ghc-flag:`-XDeriveGeneric` + extension) are still allowed. Refer to the + :ref:`generic programming <generic-programming>` section for more details. + .. _safe-overlapping-instances: Safe Overlapping Instances |