[Haskell-cafe] Ambiguous types

[Paolino]

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 <paolo.veronelli at gmail.com> wrote:

> 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
>>>
>>>
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> To (un)subscribe, modify options or view archives go to:
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>> Only members subscribed via the mailman list are allowed to post.
>>
>>
>>
>> --
>>
>> 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/>*
>
>

-- 

Paolo Veronelli (paolo.veronelli at gmail.com)


*Functional developer @ global.de <http://global.de/>*
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190807/16ff521f/attachment-0001.html>