[Haskell-cafe] How to use unsafePerformIO properly (safely?)

Emil Axelsson emax at chalmers.se
Wed Mar 31 03:00:07 EDT 2010

In Feldspar's module for observable sharing [1] I use the following

{-# OPTIONS_GHC -O0 #-}

which I assumed would take care of the steps required for 
unsafePerformIO. Could someone please tell if this assumption is correct?

(Of course, observable sharing is not safe regardless, but that's beside 
the point :) )

/ Emil


Ivan Miljenovic skrev:
> I use the dreaded unsafePerformIO for a few functions in my graphviz
> library ( http://hackage.haskell.org/packages/archive/graphviz/2999.8.0.0/doc/html/src/Data-GraphViz.html
> ).  However, a few months ago someone informed me that the
> documentation for unsafePerformIO had some steps that should be
> followed whenever it's used:
> http://www.haskell.org/ghc/docs/latest/html/libraries/base-
> .
> Looking through this documentation, I'm unsure on how to deal with the
> last two bullet points (adding NOINLINE pragmas is easy).  The code
> doesn't combine IO actions, etc. and I don't deal with mutable
> variables, so do I have to worry about them?

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