[Haskell-cafe] Using promoted lists

oleg at okmij.org oleg at okmij.org
Fri Jun 8 08:50:36 CEST 2012

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

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


for explanations.

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