Sorry, I just realized I sent the e-mail to the wrong mailing list. My bad!
On Thu, Jun 25, 2009 at 5:55 PM, Hector Guilarte <hector...@gmail.com>wrote: > Hello, > > 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 ....) > > 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? > > Thanks a lot! > > Here's the code: > > -- Tabla is my Symbol Table of the program being interpreted > evalInstruccion:: Instruccion -> Tabla -> Tabla > evalInstruccion (ShowY showY) tabla = myRunIO (evalShow showY tabla) > evalInstruccion _ tabla = tabla -- There are many other Instructions > here missing wich are not relevant to my question > > {-# NOINLINE myRunIO #-} > myRunIO:: (Tabla, IO()) -> Tabla > myRunIO tupla = ((unsafePerformIO (snd tupla)) `seq` (fst tupla)) -- Here's > the unsafePerformIO.... Am I safe? > > evalShow:: ShowY -> Tabla -> (Tabla, IO()) > evalShow (ShowS string) tabla = (tabla,(hPutStr stdout string)) > evalShow (ShowE expr) tabla = (tabla,(hPutStr stdout (show (evalExpr expr > tabla)))) -- Don't worry about evalExpr, it works and returns an Int >
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