User constructed types with new Data.Typeable and Data.Reflection

Edward Kmett ekmett at
Sat Feb 10 22:00:24 UTC 2018

Did you mean Type.Reflection?

(reflection's Data.Reflection offers a completely unrelated notion of
Typeable reflection, hence my confusion.)


On Sat, Feb 10, 2018 at 5:16 AM, Development <dev at> wrote:

> Hey guys I have a (hopefully quick) question.
> With the new `Data.Typeable` and `Data.Reflection` in base 4.10 have we
> really lost the ability for users to compose `TypeRep`’s?
> I was using `Data.Typeable` before (mainly `mkTyConApp`). But in the new
> base 4.10 I cannot find any way to achieve the same goal.
> I have read the wiki page Typeable and Typeable/BenGamari and neither
> explicitly mentions the removal of `mkTyConApp` and similar facilities. In
> fact the latter mentions potential implementations for user constructed
> type applications twice with `mkTrApp` at the beginning of the page and
> `mkApp` at the end.
> Furthermore the documentation for `Typeable` (and `Reflection`) also never
> mentions the fact that this functionality was removed.
> My question is this: Is this intentional? Is there now a consensus that
> there should not be user constructed types? Or is there some subtile issue
> that I’m missing wich prevents user constructed types for now or always?
> Thanks in advance guys.
> Best regards.
> Justus
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> ghc-devs mailing list
> ghc-devs at
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