Re: [Haskell-cafe] Re: Lazy IO and closing of file handles

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Pete Kazmier wrote:

Bryan O'Sullivan writes:

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Pete Kazmier wrote:

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I understand the intent of this code, but I am having a hard time understanding the implementation, specifically the combination of 'fix', 'flip', and 'interate'. I looked up 'fix' and I'm unsure how one can call 'flip' on a function that takes one argument.

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As to why it's okay to call "flip" on "fix" at all, look at the types involved.

fix :: (a -> a) -> a flip :: (a -> b -> c) -> b -> a -> c

By substitution:

flip fix :: a -> ((a -> b) -> a -> b) -> b

Sadly, I'm still confused. I understand how 'flip' works in the case where its argument is a function that takes two arguments. I've started to use this in my own code lately. But my brain refuses to understand how 'flip' is applied to 'fix', a function that takes one argument only, which happens to be a function itself. What is 'flip' flipping when the function passed to it only takes one argument?

fix :: (a -> a) -> a In this case, we know something about 'a': it is a function (b -> c). Substitute: fix :: ((b -> c) -> (b -> c)) -> (b -> c) Take advantage of the right-associativity of (->) fix :: ((b -> c) -> b -> c) -> b -> c Now it looks like a function of two arguments, because the return value (normally ordinary data) can in fact, in this case, take arguments. Here's another example of that: data Box a = Box a get (Box a) = a - -- get (Box 1) :: Int - -- get (Box (\a -> a)) :: Int -> Int - -- (get (Box (\a -> a))) 1 :: Int --function application is left-associative: - -- get (Box (\a -> a)) 1 :: Int - -- flip get 1 (Box (\a -> a)) :: Int Yes, it sometimes confuses me too. Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFF/vcXHgcxvIWYTTURAj5RAKCUMeAF0vosJ6ROAVlBIDHsEq/vzgCfflnR 50BmW6tuAF6mKXBtrlHdQ5Y= =uv3G -----END PGP SIGNATURE-----