See https://ghc.haskell.org/trac/ghc/ticket/8745 and https://ghc.haskell.org/trac/ghc/ticket/8827 which discuss this problem at length.
The short answer: It's conceivable that a role-unaware library author would have abstraction expectations that are defeated through the use of `coerce`.
I would strongly welcome a proposal for how to make `coerce`, and hence GeneralizedNewtypeDeriving, to be considered Safe for 7.10.
Richard
On Aug 15, 2014, at 4:04 PM, Wolfgang Jeltsch
Hi,
I would expect the function
coerce :: Coercible a b => a -> b
to be safe in the sense of Safe Haskell. However, the Data.Coerce module is marked “Unsafe”. The coerce function is also available via GHC.Exts and GHC.Prim. The former module is marked “Unsafe”, but the latter is (surprisingly) marked “Safe-Inferred”.
What are the reasons behind this?
All the best, Wolfgang
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