[Haskell-cafe] Re: How to "Show" an Operation?
Martin Drautzburg
Martin.Drautzburg at web.de
Thu Jun 10 17:38:15 EDT 2010
On Thursday, 10. June 2010 22:10:08 Maciej Piechotka wrote:
Wow!
this is somewhat above my level. I guess I need to go back to the books. I'll
document my ignorance nontheless.
> data Named a = Named String a
>
> instance Functor Named where
> f `fmap` (Named s v) = Named s (f v)
>
okay so far
> instance Applicative Named where
> pure x = Named "" x
> (Named s f) <*> (Named t v) = Named (s ++ "(" ++ t ++ ")") (f v)
Applicative. Need to study that
Control.Applicative (<*>) :: Applicative f => f (a -> b) -> f a -> f b
So in our case the Applicative is a "Named". When I apply a Named to a
function, then I get a function between the corresponding Named types. When I
pass it an Int->Char function, I get a Named Int -> Named Char function.
But here it is applied to another Named ... is that the (a->b)? Puzzeled.
> instance Eq a => Eq (Named a) where
> (Named _ x) == (Named _ y) = x == y
>
> instance Show (Named a) where
> show (Named s _) = s
>
Understood.
> namedPure :: Show a => a -> Named a
> namedPure x = Named (show x) x
When I can show something I can always name it so its name is what 'show'
would return. Okay I guess I got it. This turns a "showable" into a Named.
>
> test :: Num a
> => (a -> a) -> (a -> a) -> (a -> a) -> [String]
> test f g h = do
> [f', g', h'] <- permutations [Named "f" f, Named "g" g, Named "h" h]
According to Hoogle permutations should be in Data.List. Mine (GHCI 6.8.2)
does not seem to have it. Seems to have something to do with "base", whatever
that is.
> guard $ namedPure 42 == f' <*> g' <*> h' <*> namedPure 42
Ah, the 42 needs namedPure.
Again this <*> operator...
I believe the whole thing is using a List Monad.
> return $ show f' ++ " . " ++ show g' ++ " . " ++ show h'
I wonder if the thing returns just one string or a list of strings. I
guess "return" cannot return anything more unwrapped than a List, so it must
be a List. But does it contain just the first match or all of them? All of
them! And how many brackets are around them?
--
Martin
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