[merd-devel] upcasting (part1)

[Pixel <pi...@ma...>]

id !! x -> x
id := x -> x

to_A1 !! A -> A
to_A1 := A -> A

to_A2 !! A -> A
to_A2 := x -> x

to_A3 !! x -> (x !< A) ; x
to_A3 := x -> x


          v      v.to_A1   v.to_A2   v.to_A3    v.id

value     A         A         A         A         A   
type      A         A         A         A         A   

value  A |&| B      A      A |&| B   A |&| B   A |&| B
type      A         A         A         A         A   

value  A |&| B      A      A |&| B   A |&| B   A |&| B
type   A |&| B      A         A      A |&| B   A |&| B

value     B         _         _         _         B
type      B       error     error     error       B


- "to_A2", "to_A3" and "id" are semantically equivalent at runtime
- "to_A1" and "to_A2" have the same type, but not the same semantic