[Haskell-cafe] Fwd: Re: Overlapping instances

[Mitchell Rosen]

I see. I did know about the specificity rule, but somehow it escaped me 
that the extra y := x substitution required for the second instance to 
match means that the first instance *is* more specific.

Thanks!

PS - why do overlapping instances give you hives? Did you mean incoherent 
instances?

On Wednesday, February 24, 2016 at 7:07:54 PM UTC-8, David Feuer wrote:
>
> Oddly, this message was rejected by the system. Trying again.
> ---------- Forwarded message ----------
> From: "David Feuer" <david... at gmail.com <javascript:>>
> Date: Feb 24, 2016 10:04 PM
> Subject: Re: [Haskell-cafe] Overlapping instances
> To: "Mitchell Rosen" <mitchel... at gmail.com <javascript:>>
> Cc: "Haskell-cafe" <haskel... at googlegroups.com <javascript:>>
>
> It's possible that GHC will backtrack with overlapping enabled (I can't 
> remember, and overlapping instances give me hives) but it doesn't have 
> here. The first instance is "more specific" than the second because the 
> type variable x appears twice.
> On Feb 24, 2016 9:49 PM, "Mitchell Rosen" <mitchel... at gmail.com 
> <javascript:>> wrote:
>
>> Consider this "list membership" typeclass and two overlapping instances:
>>
>> class Elem (x :: k) (xs :: [k])
>>
>> instance {-# OVERLAPS #-} Elem x (x ': xs)
>> instance {-# OVERLAPS #-} Elem x xs => Elem x (y ': xs)
>>
>> The inductive style is satisfying, but I'm struggling to understand 
>> exactly how GHC can pick one instance over the other.
>>
>> How is a constraint like Elem Int [Int] solved? Certainly the Elem x (x 
>> ': xs) instance matches, with x = Int and xs = []. But, the second 
>> instance is also equally valid with x = Int, y = Int, xs = []. Even 
>> though *if *the second instance is chosen, the context cannot be 
>> satisfied (no instance for Elem x []), it's my understanding that GHC 
>> will not backtrack once it picks an instance. And because both instances 
>> look valid to me, I don't understand why this code does not require 
>> IncoherentInstances.
>>
>> Thanks.
>>
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