The `delabConstWithSignature` delaborator is responsible for pretty
printing constants with a declaration-like signature, with binders, a
colon, and a type. This is used by the `#check` command when it is given
just an identifier.
It used to accumulate binders from pi types indiscriminately, but this
led to unfriendly behavior. For example, `#check String.append` would
give
```
String.append (a✝ : String) (a✝¹ : String) : String
```
with inaccessible names. These appear because `String.append` is defined
using patterns, so it never names these parameters.
Now the delaborator stops accumulating binders once it reaches an
inaccessible name, and for example `#check String.append` now gives
```
String.append : String → String → String
```
We do not synthesize names for the sake of enabling binder syntax
because the binder names are part of the API of a function — one can use
`(arg := ...)` syntax to pass arguments by name. The delaborator also
now stops accumulating binders once it reaches a parameter with a name
already seen before — we then rely on the main delaborator to provide
that parameter with a fresh name when pretty printing the pi type.
As a special case, instance parameters with inaccessible names are
included as binders, pretty printing like `[LT α]`, rather than
relegating them (and all the remaining parameters) to after the colon.
It would be more accurate to pretty print this as `[inst✝ : LT α]`, but
we make the simplifying assumption that such instance parameters are
generally used via typeclass inference. Likely `inst✝` would not
directly appear in pretty printer output, and even if it appears in a
hover, users can likely figure out what is going on. (We may consider
making such `inst✝` variables pretty print as `‹LT α›` or
`infer_instance` in the future, to make this more consistent.)
Something we note here is that we do not do anything to make sure
parameters that can be used as named arguments actually appear named
after the colon (nor do we assure that the names are the correct names).
For example, one sees `foo : String → String → String` rather than `foo
: String → (baz : String) → String`. We can investigate this later if it
is wanted.
We also give `delabConstWithSignature` a `universes` flag to enable
turning off pretty printing universe levels parameters.
Closes#2846
by showing the matrix of calls and measures, and what we know about that
call (=, <, ≤, ?), e.g.
guessLexFailures.lean:27:0-33:31: error: Could not find a decreasing
measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
x1 x2 x3
1) 29:6-25 = = =
2) 30:6-23 = ? <
3) 31:6-23 < _ _
Please use `termination_by` to specify a decreasing measure
It’s a bit more verbose for mutual functions.
It will use the user-specified argument names for functions written
```
foo (n : Nat) := …
```
but not with pattern matching like
```
foo : Nat → …
| n => …
```
This can be refined later and separately (and maybe right away in
`expandMatchAltsWhereDecls`).
If here is only one plausible measure, there is no point having the
`GuessLex` code see if it
is terminating, running all the tactics, only for the `MkFix` code then
run the tactics again.
So if there is only one plausible measure (non-mutual recursion with
only one varying
parameter), just use that measure.
Side benefit: If the function isn’t terminating, more detailed error
messages are shown
(failing proof goals), located at the recursive calls.
previously, only the WellFounded code was making use of the error
location in the RecApp-metadata. We can do the same for structural
recursion. This way,
```
def f (n : Nat) : Nat :=
match n with
| 0 => 0
| n + 1 => f (n + 1)
```
will show the error with squiggly lines under `f (n + 1)`, and not at
`def f`.
