Hmm... it was my understanding that the example was showing how to *avoid* having to create a lot of functions that do the same thing but have different numbers of arguments.
>From the Wiki page: "Anytime you feel the need to define different higher order functions to accommodate for function-arguments with a different number of arguments, think about how defining a proper instance of Applicative can make your life easier." Not so? Michael --- On Thu, 8/26/10, Ivan Lazar Miljenovic <[email protected]> wrote: From: Ivan Lazar Miljenovic <[email protected]> Subject: Re: [Haskell-cafe] On to applicative To: "michael rice" <[email protected]> Cc: [email protected] Date: Thursday, August 26, 2010, 2:50 AM On 26 August 2010 16:47, michael rice <[email protected]> wrote: > > OK, fmap2 works, but not fmap3. What am I not understanding? > > Michael > > import Control.Applicative > > -- f :: (a -> b -> c) > -- fmap :: Functor f => (d -> e) -> f d -> f e > > sumsqr :: Int -> Int -> Int > sumsqr i j = i*i+j*j > > -- fmap :: Functor f => f a -> f (b -> c) -- Identify d with a, and e with > (b -> c) > > > fmap2 f a b = f `fmap` a <*> b > fmap3 f a b c = f `fmap` a <*> b <*> c > fmap4 f a b c d = f `fmap` a <*> b <*> c <*> d > > > -- fmap2 f a b = f <$> a <*> b > -- fmap3 f a b c = f <$> a <*> b <*> c > -- fmap4 f a b c d = f <$> a <*> b <*> c <*> d > > > *Main> fmap2 sumsqr (Just 3) (Just 4) > Just 25 > *Main> fmap3 sumsqr (Just 3) (Just 4) (Just 5) > > <interactive>:1:6: > Couldn't match expected type `a2 -> b' against inferred type `Int' > In the first argument of `fmap3', namely `sumsqr' > In the expression: fmap3 sumsqr (Just 3) (Just 4) (Just 5) > In the definition of `it': > it = fmap3 sumsqr (Just 3) (Just 4) (Just 5) > *Main> sumsqr takes three arguments; fmap3 has type: fmap3 :: (a -> b -> c -> d) -> Maybe a -> Maybe b -> Maybe c -> Maybe d i.e. the function you pass it needs to take 3 arguments. fmap3 (\ x y z -> z * y + z) (Just 1) (Just 2) (Just 3) > > > --- On Thu, 8/26/10, Ivan Lazar Miljenovic <[email protected]> wrote: > > From: Ivan Lazar Miljenovic <[email protected]> > Subject: Re: [Haskell-cafe] On to applicative > To: "michael rice" <[email protected]> > Cc: [email protected] > Date: Thursday, August 26, 2010, 2:33 AM > > On 26 August 2010 16:29, michael rice <[email protected]> wrote: > > > > Can you recommend an example that works? > > An example of what? > > The definitions of fmap2, etc. on that page look like they're correct. > > -- > Ivan Lazar Miljenovic > [email protected] > IvanMiljenovic.wordpress.com > -- Ivan Lazar Miljenovic [email protected] IvanMiljenovic.wordpress.com
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