This feature was originally added to implement the state monad.
It is not needed anymore since we have a better encoding that doesn't
require this feature and avoids memory allocations.
A few weeks ago, it was not feasible to inline `bind_mk_res` and
`orelse_mk_res` because the compilation time would increase a lot.
Since then I have improved the heuristics for deciding whether to float
`cases_on` or not.
So, I tried today to mark them with `@[inline]` again.
The corelib build time increased only 1.2 secs, but the `parser1.lean` runtime improved:
Before:
```
num. allocated objects: 18025367
num. allocated closures: 2988476
```
After:
```
num. allocated objects: 15774515
num. allocated closures: 2488695
```
I used my desktop to collect the numbers above.
We want them to be specialized for a given monad stack, but not
inlined. If we inline them, then every occurrence of `whitespace` and
`num` will specialize the nested `take_while?` application.
This is bad since we don't cache them.
Both `alternative` and `monad` implement `applicative`. However,
their default implementations for `seq_right` and `seq_left` are
different. The `alternative` implementation uses the inefficient default
version for `seq_right` available at `applicative`:
```
(seq_right := λ α β a b, const α id <$> a <*> b)
```
instead of the more efficient
```
(seq_right := λ α β x y, x >>= λ _, y)
```
defined at `monad` using the `bind` operator.
This commit makes sure the `applicative` instances for `reader_t`,
`state_t`, `option` and `parsec_t` use the efficient version.
I found the problem when inspecting the generated code for:
```
def symbol (s : string) : parsec' unit :=
(str s *> whitespace) <?> ("'" ++ s ++ "'")
```
@kha: I changed the specialization candidate selection procedure.
Now, instances are not considered for specializations unless we mark
them with `[specialize]`. The idea is that an instance application is
morally just creating the "dictionary" for invoking a polymorphic
function.
@kha I had to add this attribute because the specializer was generated
many specialization candidates for functions that take `[has_tokens ...]`
as an argument. Moreover, these candidates had a lot of
dependencies. I am trying to workaround this issue by marking the
instances with the new attribute `[nospecialize]`.
I did not mark instances created by `[derive]`. It is quite tedious to
do it.
BTW, when I was investigating the problem I stumbled at `node.view`.
Its type is:
```
node.view :
Π {α : Type} {m : Type → Type} [_inst_1 : monad m] [_inst_2 : monad_except (parsec.message syntax) m]
[_inst_3 : monad_parsec syntax m] [_inst_4 : alternative m] (k : syntax_node_kind) (rs : list (m syntax))
[i : @has_view syntax_node_kind k α], @has_view (m syntax) (@node m _inst_1 _inst_2 _inst_3 _inst_4 k rs) α
```
This looks wrong: the view depends on `[monad_parsec syntax m]`
We should also make sure definitions do not have unnecessary type
class instances. Otherwise, we will put additional stress on the code
specializer. One option is to change the frontend and filter unused
instances.
