[Haskell-cafe] Where do trivial functions on the usual containers live?

[Henning Thielemann]

On Sun, 20 Dec 2020, Ignat Insarov wrote:

> I am looking for a place where I can find or put some simple functions that I
> expect but do not find in the `containers` or elsewhere in the standard
> libraries.

I have my own utility library for such functions. However, it does not and 
shall not depend on 'containers'.

> Some examples I have in mind:
>
> * Group a collection by an equivalence relation:
>
>      classify ∷ Ord π ⇒ (a → π) → [a] →a]]

If the grouped elements must be consecutive then I have 
Data.List.Key.group. But I guess, that Ord means that you want to group 
all elements with the same key in one group. Then I would use Map anyway 
for representing the result.

> * From a finite map, get a map from its range to its fibers:
>
>      fibers ∷ (Ord k, Ord v) ⇒ Map k v → Map v (NonEmpty k)
>
> * Put a type into the diagonal of its square:
>
>      diagonal = λx → (x, x)

I have Data.Tuple.HT.double. But recently the name 'dup' was discussed 
here.