RE: unsafePerformIO around FFI calls

Hal Daume writes:

...

I'm curious exactly what is "safe" and what is "unsafe" to wrap unsafePerformIO around when it comes to FFI calls.

Here's a simple test:

Could you imagine an alternative implementation of the same API in pure Haskell? (Don't consider efficiency or effort required to write the implementation, just whether it can be done.)

If so, then it is ok to use unsafePerformIO and the ffi to implement the API instead.

If it fails that test, it is incredibly unlikely that it is ok and a proof that it is ok is likely to be pretty complex - maybe worth a PLDI paper or some such.

That's a nice succinct way to describe it. Another way, which boils down to the same thing but which is a little more concrete, is to ask: - Does the function's result depend only on the values of its arguments? (obviously only makes sense for a top-level IO function which you want to wrap in unsafePerformIO - for a non-top-level function or expression just replace 'arguments' with 'arguments and free variables'). Cheers, Simon