Bulat, just for the record, it's not my article. Although I have the privilege of sharing a somewhat similar name with the geniuses around here, I didn't have any part in that text. ;-) You were wondering about this declaration:
foreign import ccall unsafe sin :: Float -> Float
I guess you are confused by the difference between safe/unsafe FFI calls versus pure/impure FFI calls. Which is comforting for me, because as it happens I don't quite understand that either. ;-) I'll try to explain what I believe I do understand, and hope that the others on this list chime in case I mess something up. A pure function is one without side-effects, meaning that the result depends only on the function arguments. Those are surprisingly few; the C function strlen(3) for example is _not_ pure because it depends on the fact that the memory pointed to by the argument actually contains the string which's length you'd like to compute. So the strlen() call must occur in the correct order in relation to other I/O calls like as read(), malloc(), or whatever. Thus, the signature for strlen() would be: strlen :: Ptr CChar -> IO CSize The sin(3) function on the other hand is pure. It doesn't depend on anything but the floating point value you give it, hence in can be evaluated at any point of the program execution, hence it doesn't need to be in the IO monad: sin :: CFloat -> CFloat Now, safe and unsafe calls are a different beast: | A safe call is less efficient, but guarantees to leave the | Haskell system in a state that allows callbacks from the | external code. In contrast, an unsafe call, while carrying | less overhead, must not trigger a callback into the Haskell | system. The vast majority of C function you will call are unaware of Haskell. They will not trigger anything the Haskell runtime system needs to know about, nor will they force a part of your Haskell program to be evaluated. Those functions can be called unsafely, just like the definition above says. Since pure FFI calls don't have any side-effects, they are always safe to be called unsafely. (Yes, the choice of the words "safe" and "unsafe" is a bit unfortunate in the standard here.) So unless I am very mistaken, the declaration foreign import ccall safe sin :: Float -> Float would work just as well as the one above does, but it would be less efficient. In other words, if you declare a FFI function to be called safely, and to be called from within the IO monad, then absolutely nothing can possibly go wrong. ;-)
is it possible to download sources of http server mentioned in this article?
I think the (more or less) latest version is here: http://www.mdstud.chalmers.se/~md9ms/hws-wp/ Another somewhat extensive example for system programming in Haskell is available here: http://postmaster.cryp.to/ Hope this helps. Peter