Default onError instance in REPL

[ppolesiuk]

It would be nice to have the default instance of ~re/~onError in REPL. However the following naive definition:

data Error (effect E) = { raise : {type T} -> Unit ->[E] T }
handle {effect=RE} err = Error { raise = effect () / _ => () }
let ~onError = err.raise

have a huge drawback: when we type [].hdErr, the REPL would exit immediately. The problem can be solved at library level, by adding the following file (ReplUtils.fram) to the standard library:

pub data ReplUtils (effect E) =
  { raise  : {type T} -> Unit ->[E] T
  , commit : Unit ->[E] Unit
  }

pub handle {effect=ReplRE} repl = ReplUtils
  { raise  = effect () / _ => fn snapshot =>
    printStrLn "Runtime Error!";
    match snapshot with
    | None   => ()
    | Some f => f ()
    end
  , commit = effect () / r => fn _ =>
    let rec snapshot () = r () (Some snapshot) in
    snapshot ()
  }
  return  x => fn _ => x
  finally c => c None

let _ = repl.commit ()

pub let ~onError = repl.raise

but this would not solve all the problems:

> import List ;;
> import open ReplUtils ;;
> let foo x = x ;;
> [].hdErr ;;
: a
Runtime Error!
> foo 42 ;;
<stdin>:1:1-3: fatal error: Unbound variable foo
>

To do it correctly, we have to call repl.commit method after each REPL command, but this requires changes in the type-checker (or desugaring)