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

Development dev at
Tue Feb 13 10:49:18 UTC 2018

Ah thanks. I had no idea the `App` pattern actually was bidirectional. I had tried the `Con’` pattern but that is only valid for deconstruction.
Is there any way to tell in the docs whether a pattern is bidirectional?

> On 12 Feb 2018, at 18:30, Ben Gamari <ben at> wrote:
> Development <dev at> writes:
>> 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.
> Indeed, as is noted in the changelog for base [1], mkTyConApp and
> friends were removed. mkTyConApp in particular allowed the construction
> of ill-kinded type representations so instead of emulating the previous
> behavior we rather opted to remove it.
> If you were previously using these interfaces you almost certainly want
> to instead use the new type-indexed interface provided by
> Type.Reflection. In particular, you can use the App constructor in
> place of mkTyConApp.
> Cheers,
> - Ben
> [1]

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