[Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)

[Ryan Ingram]

Here's a way that works more closely to your original version:

instance Enumerated a => Target a where
   convert n
       | n >= 0 && n < numConstrs = Just (constrs !! n)
       | otherwise = Nothing
    where
       constrs = constructors
       numConstrs = length constrs

Alternatively:

instance Enumerated a => Target a where
   convert n
       | n >= 0 && n < numConstrs = Just result
       | otherwise = Nothing
    where
       numConstrs = length (constructors `asTypeOf` [result])
       result = constructors !! n

However let me warn you that you aren't going to be happy with this instance
when it comes time to use this.  Instead, you probably want one of the
following:

defaultConvert :: Enumerated a => Int -> a
defaultConvert n
    | n >= 0 && n < numConstrs = Just (WithEnumerated (constrs !! n))
    | otherwise = Nothing
  where
    constrs = constructors
    numConstrs = length constrs

(a)
instance Target SomeEnumeratedType where convert = defaultConvert

(b)
newtype WithEnumerated a = WithEnumerated a
instance Enumerated a => Target (WithEnumerated a) where
    convert n = WithEnumerated (defaultConvert n)

OverlappingInstances basically never does what you want in the long run.

  -- ryan

On Thu, Sep 17, 2009 at 9:01 AM, Andy Gimblett <haskell at gimbo.org.uk> wrote:

>
> On 17 Sep 2009, at 16:50, Daniel Fischer wrote:
>
>  Yes, the second appearance of 'constructors' is at an unspecified type.
>>
>> instance (Enumerated a) => Target a where
>>   convert n
>>      | n < 0     = Nothing
>>      | otherwise = case drop n constructors of
>>                       (x:_) -> Just x
>>                       _ -> Nothing
>>
>> would make it compile.
>>
>
> Neat trick.  It works: thanks!
>
>  But there'd be a risk that Target is unusable, depending on how instance
>> resolution is
>> done.
>>
>
> Unusable?  How so?  Sorry, but I don't follow...
>
>
> -Andy
>
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