Refactor the implementation of arrow notation
The implementation of arrow notation is not really satisfactory. See #1662 (closed) for some background.
To fix it properly we need to re-factor the way that arrows are typechecked. Ross and I discussed this. Our preliminary plan is this.
- Instead of re-using
HsExprfor commands, make a new data type. (C.f. the comments in
HsExprline 540 or so.)
- That will allow the renamer to decorate the (now command-specific) tree with what variables are in scope where, and that in turn will greatly simplify the desugarer. (The desugarer is currently duplicating much of what the renamer does.)
- We'll need think about constraint gathering for "holes in the scope". cf #1662 (closed)