The main motivation is that we will be able to move equalities between universes.
For example, suppose we have
A : (Type i)
B : (Type i)
H : @eq (Type j) A B
where j > i
We didn't find any trick for deducing (@eq (Type i) A B) from H.
Before this commit, heterogeneous equality as a constant with type
heq : {A B : (Type U)} : A -> B -> Bool
So, from H, we would only be able to deduce
(@heq (Type j) (Type j) A B)
Not being able to move the equality back to a smaller universe is
problematic in several cases. I list some instances in the end of the commit message.
With this commit, Heterogeneous equality is a special kind of expression.
It is not a constant anymore. From H, we can deduce
H1 : A == B
That is, we are essentially "erasing" the universes when we move to heterogeneous equality.
Now, since A and B have (Type i), we can deduce (@eq (Type i) A B) from H1. The proof term is
(to_eq (Type i) A B (to_heq (Type j) A B H)) : (@eq (Type i) A B)
So, it remains to explain why we need this feature.
For example, suppose we want to state the Pi extensionality axiom.
axiom hpiext {A A' : (Type U)} {B : A → (Type U)} {B' : A' → (Type U)} :
A = A' → (∀ x x', x == x' → B x == B' x') → (∀ x, B x) == (∀ x, B' x)
This axiom produces an "inflated" equality at (Type U) when we treat heterogeneous
equality as a constant. The conclusion
(∀ x, B x) == (∀ x, B' x)
is syntax sugar for
(@heq (Type U) (Type U) (∀ x : A, B x) (∀ x : A', B' x))
Even if A, A', B, B' live in a much smaller universe.
As I described above, it doesn't seem to be a way to move this equality back to a smaller universe.
So, if we wanted to keep the heterogeneous equality as a constant, it seems we would
have to support axiom schemas. That is, hpiext would be parametrized by the universes where
A, A', B and B'. Another possibility would be to have universe polymorphism like Agda.
None of the solutions seem attractive.
So, we decided to have heterogeneous equality as a special kind of expression.
And use the trick above to move equalities back to the right universe.
BTW, the parser is not creating the new heterogeneous equalities yet.
Moreover, kernel.lean still contains a constant name heq2 that is the heterogeneous
equality as a constant.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
After this commit, a value of type 'expr' cannot be a reference to nullptr.
This commit also fixes several bugs due to the use of 'null' expressions.
TODO: do the same for kernel objects, sexprs, etc.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
rewrite_* functions take the rewriting results of the sub-components and
construct the rewriting result for the main component.
For instance, rewrite_app function takes env, ctx, and the value v s.t.
v = (e_0 e_1 ... e_n)
and the rewriting results for e_i's as a vector(buffer)
(e'_0, pf_0 -- proof of e_0 = e'_0)
(e'_1, pf_1 -- proof of e_1 = e'_1)
...
(e'_n, pf_n -- proof of e_n = e'_n).
Then rewrite_app function construct the new v'
v' = (e'_0 e'_1 ... e'_n)
and the proof of v = v' which is constructed with pf_i's.
These functions are used in the component rewriters such as app_RW and
let_type_RW, as well as more complicated rewriters such as depth
rewriter.
The following call sequence is possible:
C++ -> Lua -> C++ -> Lua -> C++
The first block of C++ is the Lean main function.
The main function invokes the Lua interpreter.
The Lua interpreter invokes a C++ Lean API.
Then the Lean API invokes a callback implemented in Lua.
The Lua callback invokes another Lean API.
Now, suppose the Lean API throws an exception.
We want the C++ exception to propagate over the mixed C++/Lua call stack.
We use the clone/rethrow exception idiom to achieve this goal.
Before this commit, the C++ exceptions were converted into strings
using the method what(), and then they were propagated over the Lua
stack using lua_error. A lua_error was then converted into a lua_exception when going back to C++.
This solution was very unsatisfactory, since all C++ exceptions were being converted into a lua_exception, and consequently the structure of the exception was being lost.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
The printer and formatter objects are not trusted code.
We moved them to the kernel to be able to provide them as an argument to the trace objects.
Another motivation is to eliminate the kernel_exception_formatter hack.
With the formatter in the kernel, we can implement the pretty printer for kernel exceptions as a virtual method.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
