current syntax is

    exception Return (int, ghost bool) in
    ...
    try
      ...
      raise Return (5, false)
      ...
    with
      Return (i, b) -> ...
    ...

These exceptions can carry mutable and non-monomorphic values.
They can be raised from local functions defined in the scope
of the exception declaration.

Since local exceptions can carry results with type variables
and regions, we need to freeze them in the function signatures.

avoid using logical symbols in programs in a few places

I know that extraction and execution are much more fun than
the boring "can we even parse WhyML at all?" question, but
you see, if our parsing/typechecking is broken, then the fun
stuff is broken, too. And I really prefer to get the error
messages about the typechecking being broken from the typechecking
part of the bench, and not from the test-api or extraction part.
Merci pour votre compréhension.

Aesthetics is a harsh mistress.

This is a prototype version that requires no additional annotation.
In addition to the existing rules of scoping:

  - it is forbidden to declare two symbols with the same name
    in the same scope ("everything can be unambiguously named"),

  - it is allowed to shadow an earlier declaration with a newer one,
    if they belong to different scopes (e.g., via import),

we define one new rule:

  - when an rsymbol rs_new would normally shadow an rsymbol rs_old,
    but (1) both of them are either
      - binary relations: t -> t -> bool,
      - binary operators: t -> t -> t, or
      - unary operators:  t -> t
    and (2) their argument types are not the same,
    then rs_old remains visible in the current scope.
    Both symbols should take non-ghost arguments and
    return non-ghost results, but effects are allowed.
    The name itself is not taken into account, hence
    it is possible to overload "div", "mod", or "fact".

Clause (1) allows us to perform type inference for a family of rsymbols,
using an appropriate generalized signature.

Clause (2) removes guaranteed ambiguities: we treat this case as
the user's intention to redefine the symbol for a given type.

Type inference failures are still possible: either due to
polymorphism (['a -> 'a] combines with [int -> int] and
will fail with the "ambiguous notation" error), or due
to inability to infer the precise types of arguments.
Explicit type annotations and/or use of qualified names
for disambiguation should always allow to bypass the errors.

Binary operations on Booleans are treated as relations, not as
operators. Thus, (+) on bool will not combine with (+) on int.

This overloading only concerns programs: in logic, the added rule
does not apply, and the old symbols get shadowed by the new ones.

In this setting, modules never export families of overloaded symbols:
it is impossible to "use import" a single module, and have access to
several rsymbols for (=) or (+).

The new "overloading" rule prefers to silently discard the previous
binding when it cannot be combined with the new one. This allows us
to be mostly backward compatible with existing programs (except for
the cases where type inference fails, as discussed above).

This rule should be enough to have several equalities for different
program types or overloaded arithmetic operations for bounded integers.
These rsymbols should not be defined as let-functions: in fact, it is
forbidden now to redefine equality in logic, and the bounded integers
should be all coerced into mathematical integers anyway.

I would like to be able to overload mixfix operators too, but
there is no reasonable generalized signature for them: compare
"([]) : map 'a 'b -> 'a -> 'b" with "([]) : array 'a -> int -> 'a"
with "([]) : monomap -> key -> elt". If we restrict the overloading
to symbols with specific names, we might go with "'a -> 'b -> 'c" for
type inference (and even "'a -> 'b" for some prefix operators, such
as "!" or "?"). To discuss.

As we have to implement the "HERE" and "PARENT" qualifiers anyway,
allowing this shadowing lets us accept more programs without
compromising the "everything is unambigously nameable" invariant.

  (+ élimination de la preuve Coq)

The current syntax is "{| <term> |}", which is shorter than
"pure { <term> }", and does not require a keyword. Better
alternatives are welcome.

As for type inference, we infer the type pf the term under Epure
without binding destructible type variables in the program.
In particular,
  let ghost fn x = {| x + 1 |}
will not typecheck. Indeed, even if we detect that the result
is [int], the type of the formal parameter [x[ is not inferred
in the process, and thus stays at ['xi].

Another problem is related to the fact that variable and function
namespaces are not yet separated when we perform type inference.
Thus both fuctions
  let ghost fn (x: int) = let x a = a in {| x + 5 |}
and
  let ghost fn (x: int) = let x a = a in {| x 5 |}
will not typecheck, since the type of [x] is ['a -> 'a] when
we infer the type for the Epure term, but it becomes [int],
when we construct the final program expression. Probably,
the only reasonable solution is to keep variables and
functions in the same namespace, so that [x] simply can
not be used in annotations after being redefined as a
program function.

We cannot split ite's and matches created in this way,
and it is hard to control how much goes into them.
Try for now to only convert bool-valued ite's.