Thanks for answering so fast. Yes, GCL == Guarded Command Language... It is for an assigment I have in my Languages and Machines Course.
About the nicer/Haskellier solution you proposed: If there is a way of printing right in the moment the Interpreter finds the show instruction then I don't think the teacher is gonna like this soluttion (I would bet on that) so, can you (or somebody) explain a little bit better how would the ugly solution be? As I said earlier, I'm no expert in monads, actually the truth is that I know almost nothing about monads, so please, explain it to me as if you are explaining it to a Monads newbie... Also, can somebody explain me how would it be using the Writer Monad? remember is for a Monads newbie... Thanks a lot! Hector Guilarte On Thu, Jun 25, 2009 at 6:04 PM, Jochem Berndsen <joc...@functor.nl> wrote: > Hector Guilarte wrote: > > I made a GCL compiler using Alex and Happy and now I'm making the > > interpreter to that program. Here's the deal: > > > > > First of all, I'm no expert in the usage of monads. Now: > > > > Whenever a "show" instruction is found in any GCL program while the > > interpretation is being done it is supposed to print on the stdout the > > string or the aritmetic expresion it was called with, so I guessed I need > to > > run an IO operation and continue the interpretation of my program. I > managed > > to do this using unsafePerformIO and `seq` like is shown below. My > question > > is: Is it safe to use it this way? So far it is working great, but I need > to > > be sure I'm using it in a "safe" way. Like I said, I'm no expert in > monads > > and the System.IO.Unsafe documentation says: > > > > " > > *unsafePerformIO* :: > > IO< > http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html#t%3AIO > >a > > -> a > > This is the "back door" into the > > IO< > http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html#t%3AIO > >monad, > > allowing > > IO< > http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html#t%3AIO > >computation > > to be performed at any time. For this to be safe, the > > IO< > http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html#t%3AIO > >computation > > should be free of side effects and independent of its > > environment. > > " > > > > I don't know if the IO computation I'm doing is free of side effects and > > independent of its enviroment :s. (is just hPutStr stdout ....) > > Well, writing to the standard output is certainly a side effect. (This > does not mean that you cannot use unsafePerformIO. The compiler, > however, may assume that any value is free from side effects. This means > that you could get, in theory, less or more output from your program > than you want. In this sense it is not "safe".) > > > Also I've read something about my code not being executed for sure or > > something like that. Can somebody check the code and tell me if I'm > "safe" > > with it? > > It's "safe" in the sense that it probably won't blow up your computer. > It may also work. On the other hand, I would not recommend using > unsafePerformIO in this way. > > I see two possibilities for resolving this issue: > * (ugly) run your GCL (Guarded Command Language?) interpreter in the IO > monad, and using "print"/"putStr"/... whenever you encounter a 'show' > statement in the GCL program. > * (nicer/Haskellier) adapt your interpreter such that it returns a list > of Strings to output. You have then a purely functional interpreter, and > in the main function of your program you can print this list. This will > be lazily evaluated as the GCL program runs. You now have a very nice > separation of clean, pure code, and impure code in the IO monad (your > "main" function, which can be pretty small in your case). To avoid > boilerplate, you can use the Writer monad, for example, but others may > have better suggestions. > > Kind regards, > > -- > Jochem Berndsen | joc...@functor.nl > GPG: 0xE6FABFAB >
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
