@Kha We will probably have to refine the heuristic for hiding the
inaccessible names, but the first version is already useful.
Here is the error message for a `_` before this commit
```
error: don't know how to synthesize placeholder
context:
x✝⁴ : List Nat
x✝³ : Nat
x✝² : x✝⁴ ≠ []
a b x✝¹ : Nat
x✝ : [a, b] ≠ []
⊢ Nat
```
After
```
error: don't know how to synthesize placeholder
a b : Nat
: [a, b] ≠ []
⊢ Nat
```
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 I was writing the following example
```
...
induction as
| nil => cases h -- `h` is an empty type
| cons b bs ih => cases h
| head a bs => exact ⟨[], ⟨bs, rfl⟩⟩
| tail a b bs h1 => ...
```
The current `syntax` assumes the `| cons b bs ih => ...` alternative
is part of the first `cases h`. Forcing the `|` to occur in a column
>= of the corresponding `cases` is not a nice solution since it would
preven us from writing the second `cases` as I wrote it above.
That is we would have to write
```
induction as
| nil => cases h -- `h` is an empty type
| cons b bs ih =>
cases h
| head a bs => exact ⟨[], ⟨bs, rfl⟩⟩
| tail a b bs h1 => ...
```
or
```
induction as
| nil => cases h -- `h` is an empty type
| cons b bs ih => cases h
| head a bs => exact ⟨[], ⟨bs, rfl⟩⟩
| tail a b bs h1 => ...
```
I think the best solution is to use `with` when we have explicit
alternatives with `|`. The new syntax is similar to `match ... with | ...`
That is, we would write
```
...
induction as with
| nil => cases h
| cons b bs ih => cases h with
| head a bs => exact ⟨[], ⟨bs, rfl⟩⟩
| tail a b bs h1 => ...
```
@Kha I implemented the syntax for structure/class that we discussed this morning.
It is much cleaner. See new tests at `struct2.lean`.
I updated the documentation to use it.
closes#191
@Kha Note that it expands into a "let rec".
There are many other places where an optional `where`-clause is
useful. We can add them later. It is relatively easy to add support in
other places using the new helper functions
`expandWhereDeclsOpt` and `expandMatchAltsWhereDecls`
