[Haskell-cafe] bytestring, array and Safe Haskell

Austin Seipp mad.one at gmail.com
Tue May 8 22:45:18 CEST 2012

The reasoning is outlined in the user manual here:


Basically, these modules will compile without error if they were to be
compiled with -XSafe enabled. Thus, they are safe-inferred. The check
does not scrutinize individual functions in any way; all of SH works
on the level of module boundaries, as well as packages (whether or not
to enforce package trust when compiling clients.) As to why they are
safe-inferred, considering they are importing primitive
libraries/modules, and the module inlinePerformIO lives in should be
marked unsafe, well, I don't know. This is just a simple check, and
anything more powerful would likely be considerably more complex and
difficult to get right.

They should be marked as -XUnsafe since their use in a safe program
could cause crashes. It's likely not a conscious choice that this is
the case, as much as it is probably an oversight. Many of the core
libraries have needed refactoring/changes at the module level due to
Safe Haskell (and hence there is now a proliferation of *.Unsafe and
*.Safe modules, a la ForeignPtr.)

I do not know if the next major version of the ByteString library
(0.10) or array has marked these as unsafe or not. They should be if

Perhaps someone else who's more aware of the new Safe Haskell design
can comment further.

On Tue, May 8, 2012 at 3:03 PM, Francesco Mazzoli <f at mazzo.li> wrote:
> Why are
> http://hackage.haskell.org/packages/archive/bytestring/
> and
> http://hackage.haskell.org/packages/archive/array/
> Safe-inferred?
> The first one uses inlinePerformIO, so it clearly shouldn't be marked as
> Safe. Maybe Safe Haskell doesn't check that function?
> The second is a bit messier since it uses unboxed types and primitive
> operations... But they clearly should be marked as Unsafe, and it surprises
> me that Safe Haskell is that relaxed when checking for safe functions.
> Francesco.
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