That will most likely bite you since you're giving a pure type to a non-referentially transparent function. There is a reason why random functions are in a state monad or IO.
In particular, while you don't really care what order the compiler makes the underlying calls to the C function, you do care that it actually makes all the calls rather than CSEing them away. As a pure function, the compiler has every right to optimize away multiple calls to your FFI-bound random function, in favor of a single call and multiple references to the value.