Thanks to all, whom responded to this! With your help, I found my way to this, which seems to work correctly: app_len :: (Foldable f) => f a -> Int app_len = foldl' (flip ((+) . const 1)) 0 -db On Oct 30, 2015, at 2:47 PM, David Kraeutmann wrote:

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 Data.Monoid.Sum.

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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