The documentation is itself fairly funny, for those who don’t care to click ahead:
> This "function" has a superficial similarity to ‘unsafePerformIO’ but it is in fact a malevolent agent of chaos. It unpicks the seams of reality (and the IO monad) so that the normal rules no longer apply. It lulls you into thinking it is reasonable, but when you are not looking it stabs you in the back and aliases all of your mutable buffers. The carcass of many a seasoned Haskell programmer lie strewn at its feet.
Originally, that function was just called "inlinePerformIO", with the relatively tame warning of:
> Just like unsafePerformIO, but we inline it. Big performance gains as it exposes lots of things to further inlining. Very unsafe. In particular, you should do no memory allocation inside an inlinePerformIO block. On Hugs this is just unsafePerformIO
At some point, this function was moved to the newly created "Deprecated and unmentionable" group of functions with the comment
> Deprecated: If you think you know what you are doing, use unsafePerformIO. If you are sure you know what you are doing, use unsafeDupablePerformIO. If you enjoy sharing an address space with a malevolent agent of chaos, try accursedUnutterablePerformIO.
At this point, the two depreciated and unmentionable functions were actually just aliases of each other, with InlinePerformIO being depreciated, and accursedUnutterablePerformIO being merely unmentionable.
> This "function" has a superficial similarity to ‘unsafePerformIO’ but it is in fact a malevolent agent of chaos. It unpicks the seams of reality (and the IO monad) so that the normal rules no longer apply. It lulls you into thinking it is reasonable, but when you are not looking it stabs you in the back and aliases all of your mutable buffers. The carcass of many a seasoned Haskell programmer lie strewn at its feet.
> Witness the trail of destruction:
> Do not talk about "safe"! You do not know what is safe!> Yield not to its blasphemous call! Flee traveller! Flee or you will be corrupted and devoured!