[Haskell-cafe] Ambiguous types

[Paolino]

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


On Wed, 7 Aug 2019 at 13:05, Paolino <paolo.veronelli at gmail.com> 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 <kolar at fit.vut.cz> 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 at gmail.com)
>
>
> *Functional developer @ global.de <http://global.de/>*
>
>

-- 

Paolo Veronelli (paolo.veronelli at gmail.com)


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