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

[Olaf Klinke]

> 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. 
Many hackage contributors seem to maintain their own 
<maintainer>-utils
package containing bits and bobs they personally deem useful but can
not find elsewhere. I might add my own one soon. Not saying this is a
good thing, though. 

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

GHC.Exts.groupWith or Data.List.groupBy might do want you want,
depending on whether you want to aggregate globally or only consecutive
runs of equivalent elements. 

> 
> * 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)

The usual Semigroup instance of Map k v destroys data upon key
collision. For problems like this I have a newtype wrapper 

newtype SemiMap f v = SemiMap {getSemiMap :: f v}
instance (Semigroup v, Ord k) => Semigroup (SemiMap (Map k) v)

so that one can implement 'fibers' using foldMapWithKey. Is this worth
its own package? 

> 
> * Put a type into the diagonal of its square:
> 
>       diagonal = λx → (x, x)

This may be too trivial to be a library function, and its proper type
should rather be 
x -> Pair x
since we know both components have the same type. Pair is a Monad and
this function is its return. In package ghc Pair has an Applicativeinstance at least. 

Regards,
Olaf