On Tue, 5 Sep 2023, Zoran Bošnjak wrote:
The example I gave was simplified. A real problem contains several other complications which makes this simple solution not practical.
I have the package non-empty, that provides the types NonEmpty.T, Optional.T and Empty.T. With them you can encode any pattern of allowed list lengths. NonEmpty means "list length forbidden", Optional means "list length permitted", Empty means "maximum allowed length". In your example length 2, 5, 8 were allowed, so the bit pattern of allowed lengths is 001001001, which you can encode by type B0 = NonEmpty.T type B1 = Optional.T type SpecialList = B0 (B0 (B1 (B0 (B0 (B1 (B0 (B0 (B1 Empty.T)))))))) https://hackage.haskell.org/package/non-empty Maybe you can adapt this idea to your problem.