#10789

[sasa bogicevic]

Thanks!
So probably not good issue for the first PR but I will not be intimidated by the complexity.

Sasa



> On 18 Jun 2018, at 10:41, Simon Peyton Jones <simonpj at microsoft.com> wrote:
> 
> Richard is right. 
>  
> Let’s attach this useful info to the ticket, rather than ghc-devs.   I’ve done that for this exchange.
> https://ghc.haskell.org/trac/ghc/ticket/10789#comment:18 <https://ghc.haskell.org/trac/ghc/ticket/10789#comment:18>
>  
> Simon
>  
> From: ghc-devs <ghc-devs-bounces at haskell.org <mailto:ghc-devs-bounces at haskell.org>> On Behalf Of Richard Eisenberg
> Sent: 18 June 2018 04:21
> To: sasa bogicevic <brutallesale at gmail.com <mailto:brutallesale at gmail.com>>
> Cc: ghc-devs at haskell.org <mailto:ghc-devs at haskell.org>
> Subject: Re: #10789
>  
>  
> 
> 
> On Jun 17, 2018, at 10:01 AM, sasa bogicevic <brutallesale at gmail.com <mailto:brutallesale at gmail.com>> wrote:
>  
> So I guess that we need to check if one of the kinds of two types we are comparing defaults to * (or  Type if you will) and then
> add new warning that will be more descriptive as to why the failure happened. Maybe there is a way to check if what we are
> comparing are actually type families so that would make the job easier I guess.
>  
> I don't think the problem is particular to `Type` or defaulting. Instead, the problem is when one of the two mismatched types is a type family application where the type family has equations that pattern-match on an invisible parameter, and it's that invisible-parameter matching that's gone awry. Now that I think about it, detecting these particular conditions might be tricky: you might need to edit code in FamInstEnv that does type family equation lookup to return diagnostic information if a match fails. (I would look at reduceTyFamApp_maybe, and perhaps it can return something more interesting than Nothing in the failure case.) 
> 
> 
>  
> Richard Eisenberg offered some help on this but I am not sure how to grab hold of him so I'd appreciate any help I could get.
>  
>  
> Just email! :)
>  
> Thanks for looking into this!
> Richard

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