Re: [Haskell-cafe] Ambiguous types

I will try again to repair my rushing on the code side I'm interpreting that [0,0,1] -> False [0,1,2,1] -> True To remove the explicit recursion, and noticing the 0 case any (\(x:xs) -> x `elem` xs) . init . tails . filter (/= 0) -- quadratic but exit soon any identity . (zipWith (==) <*> tail) . sort . filter (/= 0) -- n log n , but has to sort always I'm not sure, what the other code should do with the accumulated part , as it accumulates duplicates too and forget about the previous boolean Hope I read it enough carefully now , to be helpful. Best On Wed, 7 Aug 2019 at 13:13, Paolino wrote:

Pardon, delete my code. I rushed the answer without reading carefully, my bad.

On Wed, 7 Aug 2019 at 13:05, Paolino wrote:

...

So about the type error, the second element of the tuple has no defined type. You can fix it substituting (pure v) with [v] if you want a list there.

Also

- chkDup ns = any (`elem` ns) - use Set.member to reduce complexity

Best

On Wed, 7 Aug 2019 at 11:53, Dušan Kolář wrote:

...

Dear Café,

I'm trying to solve a couple of examples and exercises just for me. I've come to the point, when I'm trying working code manipulating lists rewrite to work on Foldable (etc.).

Nevertheless, I must be doing some mistake, overlooking something, as simple code like this:

chkDup [] = False

chkDup (0:ns) = chkDup ns

chkDup (n:ns) = elem n ns || chkDup ns

which works fine, type-checks, can be used within other code, I'm trying to replace with more generic, but probably less efficient and wrong code:

chkDup ns = fst $ foldr f (False,mempty) ns

where

f _ res@(True,_) = res

f v res@(_,vs) = if v==0 then (False, vs) else (elem v vs, pure v <> vs)

which does not even type-check.

Nevertheless, the error message is not too helpful, searching the Internet just confirms it's wrong and that adding AllowAmbiguousTypes would not work. The error message is:

helper.hs:49:1: error:

• Could not deduce (Foldable f0)

from the context: (Eq a, Num a, Foldable t, Foldable f,

Applicative f, Monoid (f a))

bound by the inferred type for ‘chkDup’:

forall a (t :: * -> *) (f :: * -> *).

(Eq a, Num a, Foldable t, Foldable f, Applicative f,

Monoid (f a)) =>

t a -> Bool

at helper.hs:(49,1)-(53,80)

The type variable ‘f0’ is ambiguous

• In the ambiguity check for the inferred type for ‘chkDup’

To defer the ambiguity check to use sites, enable AllowAmbiguousTypes

When checking the inferred type

chkDup :: forall a (t :: * -> *) (f :: * -> *).

(Eq a, Num a, Foldable t, Foldable f, Applicative f,

Monoid (f a)) =>

t a -> Bool

|

49 | chkDup ns =

| ^^^^^^^^^^^...

So is there a nicer and working way, how to change my simple example? Is there a way, how to make it compile? Or is it useless to do that, original function is just fine...

Best regards,

Dušan

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--

Paolo Veronelli (paolo.veronelli@gmail.com)

*Functional developer @ global.de http://global.de/*

--

Paolo Veronelli (paolo.veronelli@gmail.com)

*Functional developer @ global.de http://global.de/*

-- Paolo Veronelli (paolo.veronelli@gmail.com) *Functional developer @ global.de http://global.de/*