On Fri, 04 Jun 2004 00:35:14 -0700, Ashley Yakeley <[EMAIL PROTECTED]> wrote:
In article <[EMAIL PROTECTED]>, John Meacham <[EMAIL PROTECTED]> 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
If we check that the type of "foo" is monomorphic, this is no longer unsafe.
-- Daan.
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