This PR modifies the pretty printing of anonymous metavariables to use
the index rather than the internal name. This leads to smaller numerical
suffixes in `?m.123` since the indices are numbered within a given
metavariable context rather than across an entire file, hence each
command gets its own numbering. This does not yet affect pretty printing
of universe level metavariables.
For debugging purposes, metavariables that are not defined now pretty
print as `?_mvar.123` rather than cause pretty printing to fail.
This PR improves the error messages produced by invalid projections and
field notation. It also adds a hint to the "function expected" error
message noting the argument to which the term is being applied, which
can be helpful for debugging spurious "function expected" messages
actually caused by syntax errors.
---------
Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
This PR clarifies the invalid field notation error when projected value
type is a metavariable.
Co-authored-by @sgraf812.
---------
Co-authored-by: Sebastian Graf <sg@lean-fro.org>
Where before we had
```lean
#check fun x : Nat => ?a
-- fun x ↦ ?m.7 x : (x : Nat) → ?m.6 x
```
Now by default we have
```lean
#check fun x : Nat => ?a
-- fun x => ?a : (x : Nat) → ?m.6 x
```
In particular, delayed assignment metavariables such as `?m.7` pretty
print using the name of the metavariable they are delayed assigned to,
suppressing the bound variables used in the delayed assignment (hence
`?a` rather than `?a x`). Hovering over `?a` shows `?m.7 x`.
The benefit is that users can see the user-provided name in local
contexts. A justification for this pretty printing choice is that `?m.7
x` is supposed to stand for `?a`, and furthermore it is just as opaque
to assignment in defeq as `?a` is (however, when synthetic opaque
metavariables are made assignable, delayed assignments can be a little
less assignable than true synthetic opaque metavariables).
The original pretty printing behavior can be recovered using `set_option
pp.mvars.delayed true`.
This PR also extends the documentation for holes and synthetic holes,
with some technical details about what delayed assignments are. This
likely should be moved to the reference manual, but for now it is
included in this docstring.
(This PR is a simplified version of #3494, which has a round-trippable
notation for delayed assignments. The pretty printing in this PR is
unlikely to round trip, but it is better than the current situation,
which is that delayed assignment metavariables never round trip, and
plus it does not require introducing a new notation.)
