Custom Type Error not getting triggered in the nested Type function call

Iavor Diatchki iavor.diatchki at
Wed Apr 27 17:34:10 UTC 2016


yes, this looks like a bug.  Could you please file a report on the


On Wed, Apr 27, 2016 at 9:43 AM, Magesh B <magesh85 at> wrote:

> Hello,
> I have partial type function which is invoked by another type function.
> When the inner type function fails with TypeError, outer type function is
> not been able to propagate that type error to its caller.
> As a result of it, I'm getting following error
>   • No instance for (KnownSymbol (NestedPartialTF (TypeError ...)))
>  instead of
>   • Unexpected type @ NestedPartialTF: Char
> Is this a bug? This behavior seems bit counter-intuitive as to work
> around, I had to write another (duplicate) type function at the top level
> to get the desired type error
> I'm using GHC-8 RC 3 and have attached complete src for reference.
> Regards,
> Magesh B
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