[Haskell] Re: Initialisation without unsafePerformIO

[John Meacham]

On Fri, Jun 04, 2004 at 12:35:14AM -0700, Ashley Yakeley wrote:
> In article <20040601163515.GA8357 at momenergy.repetae.net>,
>  John Meacham <john at repetae.net> wrote:
> 
> > I am a fan of allowing top level declarations of the form:
> > 
> > foo <- newIORef "foo"
> > 
> > which would behave as an initializer, with the semantics being that it
> > be evaluated at most once before foos first use. (so it could be
> > implemented via unsafePerformIO or as an init section run before main).
> > 
> > The 
> > {-# NOINLINE foo #-}
> > foo = unsafePeformIO $ newIORef "foo"
> > 
> > idiom is so common and useful, it should have some compiler support. It
> > is 'clean' too, since all we are doing is extending the "world" with new
> > state, but in a much cleaner/safer way then writing to a file or environment
> > variable or other methods of storing state in the world.
> 
> Clean it is not:
> 
> foo :: a
> foo <- newIORef undefined
> 
> writeChar :: Int -> IO ()
> writeChar x = writeIORef foo x
> 
> readString :: IO String
> readString = readIORef foo
> 
> cast :: Char -> IO String
> cast c = (writeChar c) >> readString

Yeah, such an extension would need to ensure initializers are monomorphic. another
advantage of a special syntax rather than unsafePerformIO. 
        John

-- 
John Meacham - ⑆repetae.net⑆john⑈