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 a -> a |
|
This is the "back door" into the IO monad, allowing
IO computation to be performed at any time. For
this to be safe, the IO 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