[emillon]

At runtime, GADTs are actually represented the same way as plain ADTs (that is, a tagged union). The tighter types make it possible to remove some unused branches in pattern matching, so you may remove a few tests, but memory-wise it should be the same.

[jordwalke]

In my experience, a memory win takes place because you can use something lighter weight (ADTs) to model your data as opposed to something that consumes additional memory (objects or a record of bound functions) simply to get the type system to accept your code.

Furthermore, when using ADTs to store your data (and constructing modules that operate on them) I believe the resolution to the functions that operate on them are entirely done at compile time, not run time - whether or not that is being capitalized on is a different story, but I believe it has benefits now and will have even more later as the compiler authors continue to add more optimizations.

[emillon]

Yes, exactly. ADTs are entirely known statically so there's no vtable etc.