`uint32` is a definition, and `type_checker::whnf` unfolds it.
To preserve the information at `erase_irrelevant`, we use a custom
`whnf_type` method that stops reduction as soon as a builtin runtime
type is found.
We must make sure we do not accidentally change the arity of a join
point. The arity is the number of nested lambda expressions.
For example, suppose we have
```
let jp := fun (x : unit), t
```
where `jp` is a join point of arity 1, i.e., `t` is not a lambda.
All "jumps" will be of the form: `jp ()`.
Now, suppose we simplify `t` and it becomes a lambda `fun (y : nat), y`.
We should to represent `jp` as
```
let jp := fun (x : unit) (y : nat), y
```
Because we would be changing `jp`'s arity.
We have the same problem with `cases_on` applications in LCNF.
So, we fix the problem using the same approach: an auxiliary
`let`-declaration. The simplified join point above is encoded as
```
let jp := fun (x : unit),
let _z := fun (y : nat), y
in _z
```
cc @kha This is the bug that I mentioned on slack :)
@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.
