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

Travis Cardwell travis.cardwell at extrema.is
Thu Jun 3 09:05:54 UTC 2021

On Thu, Jun 3, 2021 at 4:49 PM Henning Thielemann wrote:
> On Thu, 3 Jun 2021, Travis Cardwell wrote:
> > 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?

Indeed they are.  I use the following directive to hide the warnings for
that module:

    {-# OPTIONS_GHC -fno-warn-orphans #-}

> > 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" ...?

In cases where the formats are fixed and finite/few, one could create
a newtype corresponding to each format.  Personally, I think that using
a formatting function would result in a better design in most cases,

For example, one might want to format numbers according to locale, using
appropriate decimal separators.  If newtypes are defined for each
supported format, the constructor could be passed to functions to
determine how values are formatted.

      :: TTC.Render a
      => (Quantity -> a)
      -> [Quantity]
      -> TLB.Builder

In my opinion, passing a formatting function instead results in a better
design because it is more flexible.

      :: (Quantity -> TLB.Builder)
      -> [Quantity]
      -> TLB.Builder

This function might be called as follows, where locale is a sum type
that enumerates the supported locales (instead of using newtypes) and
the formatFor function formats the Quantity value appropriately for the
specified locale:

    formatQuantityList (Quantity.formatFor locale) quantities

I hope that I understood your question correctly and that my response is

Best regards,


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