[Haskell-cafe] Determining the length of a Foldable Applicative.
David Kraeutmann
kane at kane.cx
Fri Oct 30 21:56:29 UTC 2015
Your types don't match here, but you have the right idea.
'sum' won't be enough. Take a look at the definition of sum:
> sum :: (Foldable t, Num a) => t a -> a
> sum = getSum . foldMap Sum
That won't work, since that requires '(Num a)'.
We just want to count 1 up whenever we map an element.
Try doing something with const and Sum.
(Forgot to reply to list)
On 10/30/2015 9:03 PM, David Banas wrote:
> Hi all,
>
> I thought I had a simple way to determine the “length" (i.e. - number of elements in) of a Foldable Applicative container:
>
> import Prelude hiding (sum)
> import Data.Foldable (Foldable(..), sum)
> import Control.Applicative
>
> -- Calculate the "length" (i.e. - number of elements in) an Applicative container.
> app_len :: (Applicative f, Foldable f) => f a -> Int
> app_len = sum $ pure 1
>
> but I didn’t:
>
> app_len_test.hs:9:11:
> Could not deduce (Foldable t0) arising from a use of ‘sum’
> from the context (Applicative f, Foldable f)
> bound by the type signature for
> app_len :: (Applicative f, Foldable f) => f a -> Int
> at app_len_test.hs:8:12-52
> The type variable ‘t0’ is ambiguous
> Note: there are several potential instances:
> instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
> instance GHC.Arr.Ix i => Foldable (GHC.Arr.Array i)
> -- Defined in ‘Data.Foldable’
> instance Foldable (Const m) -- Defined in ‘Data.Foldable’
> ...plus four others
> In the expression: sum
> In the expression: sum $ pure 1
> In an equation for ‘app_len’: app_len = sum $ pure 1
>
> app_len_test.hs:9:17:
> Could not deduce (Applicative t0) arising from a use of ‘pure’
> from the context (Applicative f, Foldable f)
> bound by the type signature for
> app_len :: (Applicative f, Foldable f) => f a -> Int
> at app_len_test.hs:8:12-52
> The type variable ‘t0’ is ambiguous
> Note: there are several potential instances:
> instance Data.Monoid.Monoid a => Applicative ((,) a)
> -- Defined in ‘Control.Applicative’
> instance Applicative ((->) a) -- Defined in ‘Control.Applicative’
> instance Control.Arrow.Arrow a =>
> Applicative (Control.Arrow.ArrowMonad a)
> -- Defined in ‘Control.Applicative’
> ...plus 14 others
> In the second argument of ‘($)’, namely ‘pure 1’
> In the expression: sum $ pure 1
> In an equation for ‘app_len’: app_len = sum $ pure 1
>
> Can anyone help me understand what I’m missing?
>
> Thanks, and have a great weekend,
> -db
>
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