This PR refines several error error messages, mostly involving invalid
use of field notation, generalized field notation, and numeric
projection. Provides a new error explanation for field notation.
## Error message changes
In general:
- Uses a slightly different convention for expression-type pairs, where
the expression is always given `indentExpr` and the type is given
`inlineExpr` treatment. This is something of a workaround for the fact
that the `Format` type is awkward for embedding possibly-linebreaking
expressions in not-linebreaking text, which may be a separate issue
worth addressing.
- Tries to give slightly more "why" reasoning — the environment does not
contain `String.parse`, and _therefore you can't project `.parse` from a
`String`_.
Some specific examples:
### No such projection function
```lean4
#check "".parse
```
before:
```
error: Invalid field `parse`: The environment does not contain `String.parse`
""
has type
String
```
after:
```
error: Invalid field `parse`: The environment does not contain `String.parse`, so it is not possible to project the field `parse` from an expression
""
of type `String`
```
### Type does not have the correct form
```lean4
example (x : α) := (foo x).foo
```
before:
```
error: Invalid field notation: Type is not of the form `C ...` where C is a constant
foo x
has type
α
```
after:
```
error: Invalid field notation: Field projection operates on types of the form `C ...` where C is a constant. The expression
foo x
has type `α` which does not have the necessary form.
```
## Refactoring
Includes some refactoring changes as well:
- factors out multiple uses of number (1, 2, 3, 212, 222) to ordinal
("first", "second", "third", "212th", "222nd") conversion into
Lean.Elab.ErrorUtils
- significant refactoring of `resolveLValAux` in `Lean.Elab.App` — in
place of five helper functions, a special-case function case analysis,
and a case analysis on the projection type and structure, there's now a
single case analysis on the projection type and structure. This allows
several error messages to be more explicit (there were a number of cases
where index projection was being described as field projection in an
error messages) and gave the opportunity to slightly improve positining
for several errors: field *notation* errors should appear on `foo.bar`,
but field *projection* errors should appear only on the `bar` part of
`foo.bar`.
This PR improves the error message encountered in the case of a type
class instance resolution failure, and adds an error explanation that
discusses the common new-user case of binary operation overloading and
points to the `trace.Meta.synthInstance` option for advanced debugging.
## Example
```lean4
def f (x : String) := x + x
```
Before:
```
failed to synthesize
HAdd String String ?m.5
Hint: Additional diagnostic information may be available using the `set_option diagnostics true` command.
```
After:
```
failed to synthesize instance of type class
HAdd String String ?m.5
Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
Error code: lean.failedToSynthesizeTypeclassInstance
[View explanation](https://lean-lang.org/doc/reference/latest/find/?domain=Manual.errorExplanation&name=lean.failedToSynthesizeTypeclassInstance)
```
The error message is changed in three important ways:
* Explains *what* failed to synthesize, using the "type class"
terminology that's more likely to be recognized than the "instance"
terminology
* Points to the `trace.Meta.synthInstance` option which is otherwise
nearly undiscoverable but is quite powerful (see also
leanprover/reference-manual#663 which is adding commentary on this
option)
* Gives an error explanation link (which won't actually work until the
next release after this is merged) which prioritizes the common-case
explanation of using the wrong binary operation
This PR adds error explanations for two common errors caused by large
elimination from `Prop`. To support this functionality, "nested" named
errors thrown by sub-tactics are now able to display their error code
and explanation.
This PR updates the formatting of, and adds explanations for, "unknown
identifier" errors as well as "failed to infer type" errors for binders
and definitions.
It attempts to ameliorate some of the confusion encountered in #1592 by
modifying the wording of the "header is elaborated before body is
processed" note and adding further discussion and examples of this
behavior in the corresponding error explanation.
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 adds explanations for a few errors concerning noncomputability,
redundant match alternatives, and invalid inductive declarations.
These adopt a lower-case error naming style, which is also applied to
existing error explanation tests.
