[Haskell-cafe] Re: Curiose types

[Jon Fairbairn]

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Andrew Coppin <andrewcoppin at btinternet.com> writes:

> As I lay in bed last night, a curios fact occurred to
> me. (Yes, I don't get out very much...)

You probably ought to get out of bed from time to time, you
know.

> Consider the map function:
> 
>   map :: (a -> b) -> [a] -> [b]
> 
> There are two ways you can think about this function. First,
> you can see it as meaning
> 
>   map :: (a -> b) -> ([a] -> [b])
> 
> Which is beautifully symmetric. Alternatively, you can think
> about how you actually use it:
> 
>   map :: ((a -> b) -> [a]) -> [b]

No, if you think like that, you're wrong! That would be a
function that takes an object of type ((a->b) -> [a]) and
returns a [b] (which if you think about it, would be an odd
sort of function). (->) associates to the right.



-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk