Spectral/Notes/known_bugs.txt

A list of bugs and/or unintuitive behavior in Lean 2:

- When using the "have" or "assert" tactic, no coercion is applied to the type. So you have to write for example
`have g : Group.carrier G, from _,`
instead of
`have g : G, from _,`

- coercions are still displayed by the pretty printer

- When using the calc mode for homotopies, you have to give the proofs using a tactic (e.g. `by exact foo` instead of `foo`)

- A file named "module.hlean" cannot be imported using `import .module` because `module` is a keyword (but it can be imported using `import ..foo.module`)

Issues which are not bugs, but still good to know

- once you start tactic mode, you cannot specify universe levels anymore

- esimp is slow, and runs out of memory easily. It is good behavior to split up definitions. So instead of
```
equiv.MK (* big function *)
         (* big inverse *)
         (* long proof *)
         (* long proof *)
```
first write the functions f and g and then write
```
equiv.MK f
         g
         abstract (* long proof *) end
         abstract (* long proof *) end
```
this has the additional advantage that if f and/or g are defined using induction, they will only reduce if they are applied to arguments for which they actually reduce (assuming they have the correct [unfold n] flag.

- unfold [foo] also does various (sometimes unwanted) reductions (as if you called esimp)

- The Emacs flycheck mode has a hard time with nonrelative paths to files in the same directory. This means that for importing files from the Lean 2 HoTT library use absolute paths (e.g. `import algebra.group`) and for importing files in the Spectral repository use relative paths (e.g. for a file in the `homotopy` folder use `import ..algebra.subgroup`)

- The induction tactic doesn't fail if it fails to solve the type class constraint. This means that it will accept the tactics until the end of the tactic proof, when it raises the error that the term has unsolved metavariables

- Sometimes the lean-server doesn't give information anymore (and then it causes tab complete to hang). In this case, execute `M-x lean-server-restart-all-processes`. You can stop tab-complete from hanging by pressing `C-g` multiple times, once for each time you pressed TAB.