@Kha the pretty printer fails when we use the `fun+match` macro. Example:
```
fun
| PSum.inl a => 1 + sizeof a
| PSum.inr b => 1 + sizeof b
```
The test `Reparen.lean` fails without this commit. Here is the error message
```
error: no known parenthesizer for kind 'Lean.Parser.Term.matchAlts'
```
@Kha I cannot change the name is one step. The problem is that stage0
contains only the function `l_Lean_Name_Name_beq___boxed`.
So, I have to add a function with the new name, update stage0, and
then delete the old one. Otherwise, the build breaks because
the interpreter will fail when we try to test whether two names are
equal or not. The summary is: we cannot change the Lean name for a
primitive in one step because it changes the name of the auxiliary
"boxed" definition used by the interpreter.
We use `MProd` instead of `Prod` to group values when expanding the
`do` notation. `MProd` is a universe monomorphic product.
The motivation is to generate simpler universe constraints in code
that was not written by the user but generated by the `do` macro.
Note that we are not really restricting the macro power since the
`HasBind.bind` combinator already forces values computed by monadic
actions to be in the same universe.
The new test cannot be compiled without this modication.
@Kha `withReader` is a well-behaved version of `adaptReader`. `adaptReader` is
too general, and it often produces counterintuitive elaboration
errors.
Here are two super annoying issues I hit all the time:
1- `adaptReader` + polymorphic code
```
def ex1 : ReaderT Nat IO Unit :=
adaptReader (fun x => x + 1) $
IO.println "foo" -- 3 Errors here failed to synthesize `Monad ?m` and `MonadIO ?m`, and don't know how to synthesize `Type → Type`
```
2- `adaptReader` and notation that requires the expected type
```
structure Context :=
(x y : Nat)
def ex2 : ReaderT Context IO Nat :=
adaptReader (fun s => { s with x := 10 }) $ -- Error at the structure instance
...
```
In the example above, I have to write `fun (s : Context) => ...` to
fix the problem.
The two problems above happen in the old and new frontends. However,
there is a new problem specific for the new frontend. In the new
frontend, a `do` is only elaborated when the expected type is known.
So, `adaptReader (fun ctx => ...) do ...` seldom works :(
As I said above, the issue is that `adaptReader` is too general. Its
type is
```
{ρ ρ' : Type u_1} → {m m' : Type u_1 → Type u_2} → [MonadReaderAdapter ρ ρ' m m'] → {α : Type u_1} → (ρ' → ρ) → m α → m' α
```
`withReader` is a simpler version of `adaptReader`
```
withReader : {ρ : Type u_1} → {m : Type u_1 → Type u_2} → [MonadWithReader ρ m] → {α : Type u_1} → (ρ → ρ) → m α → m α
```
It doesn't have any of the problems above. Moreover, I managed to replace
every single instance of `adaptReader` with `withReader` at the stdlib
and tests. We don't need the `adaptReader` generality.
