[Haskell-beginners] How to best handle classes of types, interactions and listing of different types?

[Silent Leaf]

By the way if anyone knows the price on performances and any other
technical or non-obvious price that one must pay while using
ExistentialQuantificators, esp to make heterogeneous types then lists? On
the subject of all those half-extensions half-hacks (?) i sometimes really
hesitate, fearing some optimisations will suddenly disappear just because
of one or two bad uses of those extensions or another.

In fact with my problem i have the distinct intuition that it seems really
absurd to have to use extensions of the basic haskell just to solve a
situation that to me seems very possibly usual, or at least far from rare.
I keep thinking I took the problem wrongly, or I'm just not seeking to do
the right thing, but so far zippo...
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