[Haskell-cafe] [ANN] ttc- - Textual Type Classes

Henning Thielemann lemming at henning-thielemann.de
Thu Jun 3 07:49:49 UTC 2021

On Thu, 3 Jun 2021, Travis Cardwell wrote:

> On Thu, Jun 3, 2021 at 2:57 PM Henning Thielemann wrote:
>> That is, your class is intended for text representation for program users
>> (instead of programmers)?
> Yes, that is correct.  While Show instances should produce valid Haskell
> code and Read instances should parse that code to create the original
> value, forming an isomorphism, the Render and Parse type classes can be
> used by the developer to define instances as required for each
> particular application.  The Render and Parse type classes in Data.TTC
> have no instances, allowing developers to write their own instances for
> Int for example, but some default instances can optionally be imported
> from Data.TTC.Instances when they are appropriate.

Are these instances orphan?

> The common way to get around the one-instance-per-type limitation is to 
> use newtypes.  I generally do not use this strategy with TTC to provide 
> different ways to format text, however.

Is it feasible to have a newtype for any of "%3d", "%03d", "%05d" ...?

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