Proposal: Use (.) and id from Control.Category in Prelude

Sjoerd Visscher sjoerd at
Sat Jun 14 21:10:26 UTC 2014

I strongly disagree, this would benefit all Haskell users, and will improve error messages!

Packages like lens now jump through hoops to be able to do composition with the Prelude (.), making proper abstraction impossible, resulting in horrible error messages spewing the internals of the library to the unsuspecting user.

Composition is very important for modular code, and should be made as accessible to all users as possible.


On 14 Jun 2014, at 22:31, Johan Tibell <johan.tibell at> wrote:

> On Sat, Jun 14, 2014 at 10:15 PM, Carter Charbonneau <zcarterc at> wrote:
> (.) and id are generalized in the Category class in Control.Category. In the past, many libraries have instructed users to use (.) and id from Control.Category for composition of their types. Importing these types into Prelude shouldn't break existing code, and makes libraries that use Category easier to use.
> Discussion period: 4 weeks
> -1
> This would benefit very few Haskell users and make error messages terrible.
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