[Haskell-beginners] question from Yet Another Haskell Tutorial
Michael Mossey
mpm at alumni.caltech.edu
Sun Mar 29 09:32:04 EDT 2009
Daniel Fischer wrote:
> A simple way to check for sanity of the results is:
>
> Prelude> :t flip foldr 0 . flip. curry
> flip foldr 0 . flip. curry :: (Num c) => ((c, b) -> c) -> [b] -> c
>
> Prelude> :t \f list -> foldr (\x y -> f (y,x)) 0 list
> \f list -> foldr (\x y -> f (y,x)) 0 list :: (Num b) =>
> ((b, a) -> b) -> [a] -> b
>
> Prelude> :t \f -> foldr (uncurry $ flip f) 0
> \f -> foldr (uncurry $ flip f) 0 :: (Num b1) =>
> (b -> a -> b1 -> b1) -> [(a, b)] -> b1
>
> So you see that your result has the correct type, while Hal's hasn't.
>
>> Can anyone figure out what Hal is up to, or does it look like a simple
>> mistake?
>
> Simple mistake. YAHT was never systematically proofread to catch all
> such mistakes, so there are several still in. Nevertheless, it is an
> excellent tutorial.
Thanks for the information, glad to know I was correct.
I agree that YAHT is excellent... I am working from several books and
tutorials simultaneously to get various perspectives, and YAHT covers
much of the same material as the books, but in a more compact form, with
good exercises.
-Mike
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