[Haskell-cafe] Using promoted lists

[oleg at okmij.org]

Yves Pare`s wrote:
> So I'm trying to make a type level function to test if a type list contains
> a type. Unless I'm wrong, that calls to the use of a type family.

More crucially, list membership also calls for an equality
predicate. Recall, List.elem has the Eq constraint; so the type-level
membership test should have something similar, a similar equality
predicate. As you have discovered, type equality is quite non-trivial.

All is not lost however. If we `register' all the types or type
constructors, we can indeed write a type-level membership function. In
fact,
	http://okmij.org/ftp/Haskell/TTypeable/ShowListNO.hs

does exactly that. It uses the type-level (and higher-order) 
function Member to test if a given type is a one of the chosen types. If it
is, a special instance is selected. Please see

	http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable
	http://okmij.org/ftp/Haskell/typeEQ.html#without-over

for explanations.