[GHC] #8984: Improve output of failed GeneralizedNewtypeDeriving coercion due to type roles

[GHC]

#8984: Improve output of failed GeneralizedNewtypeDeriving coercion due to type
roles
--------------------------------------------+------------------------------
        Reporter:  haasn                    |            Owner:
            Type:  feature request          |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.8.1
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Description changed by simonpj:

Old description:

> When trying to build acme-schoenfinkel (as an example), I came across an
> error like this:
>
> {{{
> Control/Category/Schoenfinkel.hs:66:48:
>     Could not coerce from ‘cat (cat b c,
>                                 b) c’ to ‘cat (WrappedSchoenfinkel cat b
> c, b) c’
>       because ‘cat (cat b c, b) c’ and ‘cat (WrappedSchoenfinkel cat b c,
>                                              b) c’ are different types.
>       arising from the coercion of the method ‘app’ from type
>                    ‘forall b c. cat (cat b c, b) c’ to type
>                    ‘forall b c.
>                     WrappedSchoenfinkel cat (WrappedSchoenfinkel cat b c,
> b) c’
>     Possible fix:
>       use a standalone 'deriving instance' declaration,
>         so you can specify the instance context yourself
>     When deriving the instance for (ArrowApply
>                                       (WrappedSchoenfinkel cat))
> }}}
>
> It was not immediately clear to me as a user that this was caused due to
> it wanting GNT which was now being blocked by type roles (rightfully so,
> as this instance allows UnsafeCoerce!), and beyond that, what the
> specific rule that triggered this was.
>
> In this case the failure is due to `WrappedSchoenfinkel cat b c` not
> being nominally equal to `cat b c`, in the type `(WrappedSchoenfinkel cat
> b c, b)`, which is required to be nominally equal to `(cat b c, b)`
> because it's used as a parameter to the variable `cat`.
>
> Printing a location breakdown and context information similar to this to
> the user would help understanding and debugging roles a lot.

New description:

 When trying to build acme-schoenfinkel (as an example), I came across an
 error like this:

 {{{
 Control/Category/Schoenfinkel.hs:66:48:
     Could not coerce from ‘cat (cat b c,
                                 b) c’ to ‘cat (WrappedSchoenfinkel cat b
 c, b) c’
       because ‘cat (cat b c, b) c’ and ‘cat (WrappedSchoenfinkel cat b c,
                                              b) c’ are different types.
       arising from the coercion of the method ‘app’ from type
                    ‘forall b c. cat (cat b c, b) c’ to type
                    ‘forall b c.
                     WrappedSchoenfinkel cat (WrappedSchoenfinkel cat b c,
 b) c’
     Possible fix:
       use a standalone 'deriving instance' declaration,
         so you can specify the instance context yourself
     When deriving the instance for (ArrowApply
                                       (WrappedSchoenfinkel cat))
 }}}

 It was not immediately clear to me as a user that this was caused due to
 it wanting GNT which was now being blocked by type roles (rightfully so,
 as this instance allows `UnsafeCoerce`!), and beyond that, what the
 specific rule that triggered this was.

 In this case the failure is due to `WrappedSchoenfinkel cat b c` not being
 nominally equal to `cat b c`, in the type `(WrappedSchoenfinkel cat b c,
 b)`, which is required to be nominally equal to `(cat b c, b)` because
 it's used as a parameter to the variable `cat`.

 Printing a location breakdown and context information similar to this to
 the user would help understanding and debugging roles a lot.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8984#comment:3>
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