Optimisation and unsafePerformIO

David Sabel dsabel@stud.uni-frankfurt.de
Mon, 28 Oct 2002 12:31:23 +0100

Of course, I used unsafePerformIO in an unsafe way!
I'm thinking about a way to make unsafePerformIO safe.
Therefore the compiler can't do any transformation the ghc does and I want
to locate these transformations.

----- Original Message -----
From: "Simon Marlow" <simonmar@microsoft.com>
To: "David Sabel" <dsabel@stud.uni-frankfurt.de>;
Sent: Monday, October 28, 2002 10:41 AM
Subject: RE: Optimisation and unsafePerformIO

> Consider the following program:
> ---------------------------------
> {-# NOINLINE b #-}
> b x  = if even x then unsafePerformIO getChar else bot
> bot = bot
> main = do
>          putChar (b 4)
>          putChar (b 6)
> -----------------------------
> when you compile the programm with the options: -O0
> and execute the program you get:
> > test
> ab      (That's the input)
> ab      (That's the ouput)
> when you compile the programm with the options: -O1 -fno-cse
> you get:
> > test
> ab
> aa

You are using unsafePerformIO in an unsafe way.  The meaning of your
program depends on whether the compiler implements full laziness or not,
which is a decision left entirely up to the compiler implementor.  If
you want to write portable code, don't use unsafePerformIO in this way.

What exactly is it you're trying to achieve?  Perhaps we can suggest a
better solution.