@Kha This one required a bunch of manual fixes. The main issue is that
before we added the string interpolation feature, we created
`MessageData`s using `++` and coercions. For example, given
`(e : Expr)`, we would write
```
let msg : MessageData := "type: " ++ e
```
and rely on the coercions `String -> MessageData` and
`Expr -> MessageData`, and the instance `Append MessageData`.
However, heterogeneous operators "block" the expected type propagation downwards.
This kind of code is obsolete now since we can write a more compact
version using string interpolation
```
let msg := m!"type: {e}"
```
We need `MetaM` methods such as `isProp` to improve `ppGoal`.
This commit also moves `currNamespace` and `openDecls` to
`Core.Context`. Without this change, `Meta.ppExpr` was not taking
`open` commands into account.
@Kha I was tired of writing `arbitrary _` :)
There 0 places in the stdlib where the type needs to be provided.
If in the future we need to specify the type we can use
`arbitrary (α := <type>)`
@Kha Not sure whether we should have an option for supressing this
information or not.
We need this information for diagnosing problems. For example, I was trying to
understand why the elaborator was looping. I suspected it was the
TC module, but I was not getting any trace messages since the symbol
was overloaded, and the case that did not work was the expensive one :(
@Kha I am also tracking `currNamespace` and `openDecls`.
BTW, I also tried an experiment where I added `currNamespace` and
`openDecls` to `Meta.Context`, but it looked weird. This information
is only needed in the elaborator and pretty printer.
The `PPContext` object should contain everything you need. You
can put `currNamespace` and `openDecls` in the `Delaborator.Context`.
@Kha I am using `_shadowed.<idx>` suffix for marking variables that
have been shadowed. It is a bit verbose, but at least it is easy to
understand understand error messages such as
```
shadow.lean:4:0: error: type mismatch
h
has type
x._shadowed.1 = x._shadowed.1
but it is expected to have type
x = x
```
It is better than the old cryptic version
```
shadow.lean:4:0: error: type mismatch
h
has type
x = x
but it is expected to have type
x = x
```
