unsafePerformIO

[George Russell]

Sven Panne wrote:
> Huh? I'm not sure what you mean exactly, but with the help of unsafePerformIO
> and a pragma for clever compilers you can simulate something like a global
> variable in Haskell. Here an excerpt from the GLUT menu handling module:
>
>     {-# NOINLINE theMenuTable #-}
>     theMenuTable :: IORef MenuTable
>     theMenuTable = unsafePerformIO (newIORef emptyMenuTable)
...
> 
> Definitely not something to be proud of, but quite handy from time to time. :-)
> Alternatives are passing the IORef to every function which needs it (but this
> would clutter up the GLUT API in the above case) or using the FFI for holding
> global data on the C side. GHC uses a similar hack internally, too, BTW.

This is not just handy from time to time, but very frequently.  I've
just discovered it occurs about 100 times in 100K LOC that I am
responsible for, so often that I have stopped feeling guilty about it.
I don't really know what else I could do.  One solution would be to use
implicit parameters to pass a global state variable around, but this would
require me to change the type of huge numbers of functions, since implicit
parameters are still explicit in types.

I don't remember having any actual bugs caused by unsafePerformIO.  Well,
only one, before I knew about putting NOINLINE in.