@Kha I am calling it `ST` for lack of a better name. It makes some
sense since only the `IO.Ref` operations are in `EIO Empty` :)
That being said, it may confuse Haskell users.
BTW, I had to give the name to avoid a nontermination in the TC
procedure when using
```lean
instance EIOEmpty.monadLift {ε} : HasMonadLift (EIO Empty) (EIO ε) :=
{ monadLift := fun α => fromEmptyEIO }
```
`reset` was used to implement a "buggy" `IO.ref.modify`.
```lean
@[inline] def Ref.modify {α : Type} (r : Ref α) (f : α → α) : m Unit := do
v ← r.get;
r.reset;
r.set (f v)
```
`IO.Ref.reset` will store a nullptr in `r`.
Now, suppose another thread tries to read this reference,
it will get an `IO.error`.
It is not a crash, but it is really weird behavior.
Otherwise, we would parenthesize `1 + 2 + 3` as `(1 + 2) + 3` since in
```
@[builtinTermParser] def add := tparser! infixL " + " 65
```
there is an additional `checkPrec 1024` implied by `tparser!`. Now `contPrec` of this parser is (min 1024 65 = 65).
Otherwise, we would parenthesize `... : α → β := ...` as `... : (α → β) := ...` since in
```
def declValSimple := parser! " := " >> termParser
```
there is an implicit `checkPrec 1024` from `parser!`.
We would need to add the following instance to fix this example
```lean
instance ExceptT.monadIO {σ} (m : Type → Type) [Monad m] [MonadIO m] : MonadIO (ExceptT σ m) := {}
```
However, this instance may create problems for polymorphic functions on
`MonadIO m`. If `m` is of the form `ExcepT Except1 ... EIO Exception2`,
then `finally` and `catch` is catching `Except1`. This is particularly
bad for code that uses `finally`.
Right now, we are not using monad stacks with multiple `ExceptT`s for
performance reasons, but we should have a solution for users.
@Kha I am reverting this change for now.
I understand that the "default-value" approach is bad for debugging,
and it does not produce good error messages, but at least the frontend
will not "panic" when users add a bad macro.
After we switch to the new frontend, we can have a monadic `getArg`
and `getArgs` in the Elab and Macro monads which produces an
"unexpected syntax" error message. I say we wait for the new frontend
because we will be able to write `(<- s.getArg)` inside of
expressions.
