make Data.DList trustworthy

[int-e]

Safety of Data.DList is not inferred because of the use of GHC.Exts,
but the IsList class is safe to use, so the module can be trusted.

[int-e]

I guess I have not explained why this is useful. Safe Haskell provides a mechanism for delivering on the promise that pure functions don't have side effects nor break the type system, by marking functions like unsafePerformIO or unsafeCoerce that break this promise as unsafe, and disallowing the use of unsafe functions in Safe code. This is useful for sandboxes like lambdabot's Haskell evaluation.

Unfortunately, this is done on a module level, so the GHC.Exts module is unsafe (for example, it exports unsafeCoerce#) is unsafe. The way it's used in Data.DList is perfectly safe though. Safe Haskell allows marking modules that are safe despite importing unsafe modules as TrustWorthy. If the package is trusted, then trustworthy modules can be used by Safe code.

So this patch serves a niche purpose but it also should not incur any maintenance effort, whereas currently anyone running a lambdabot instance who wants to provide access to dlist has to patch the dlist package.

[spl]

Apologies for the delayed response. This will be released in v0.8.0.4.