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

Here's what happens: fix has type (x->x)->x and that has to match the first argument to flip, namely 'a->b->c'. The only chance of that is if x is actually a function type. Pick x=b->c, now we have fix has type ((b->c)->b->c)->b->c and it matches a->b->c if a=(b->c)->b->c Flip returns b->a->c, and if we substitute we get b->((b->c)->b->c)->c If you rename the variables you get the suggested type. -- Lennart On Mar 19, 2007, at 20:35 , 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.

...

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?

Thanks, Pete

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