'with' expression body type and exception handling

[aav]

The compiler adds None type to the 'with' statement body type. (U8 | None) in the example below, although there is no path which may lead to value of type 'None'

The following code

class Disposable
  new create() => None
  fun dispose() => None
  fun value(): U8 => 42

primitive TestWith
  fun apply(): U8  =>
    with d = Disposable do 
      d.value()
    end

Leads to

aav@aav ~/p/p/withNone (master)> ponyc
Building builtin -> /usr/local/Cellar/ponyc/0.10.0/packages/builtin
Building . -> /Users/aav/pony-protobuf/pony/withNone
Error:
/Users/aav/pony-protobuf/pony/withNone/with_none.pony:8:5: function body isn't the result type
    with d = Disposable do
    ^
    Info:
    /Users/aav/pony-protobuf/pony/withNone/with_none.pony:7:16: function return type: U8 val
      fun apply(): U8  =>
                   ^
    /Users/aav/pony-protobuf/pony/withNone/with_none.pony:4:16: function body type: (U8 val | None val^)
      fun value(): U8 => 42
                   ^
    /usr/local/Cellar/ponyc/0.10.0/packages/builtin/none.pony:1:1: None val^ is not a subtype of U8 val
    primitive None is Stringable
    ^
    /usr/local/Cellar/ponyc/0.10.0/packages/builtin/none.pony:1:1: not every element of (U8 val | None val^) is a subtype of U8 val
    primitive None is Stringable
    ^

At the same type, adding 'else' section to the 'with' statement leads to "try expression never results in an error"

primitive TestWith
  fun apply(): U8  =>
    with d = Disposable do 
      d.value()
    else
      0
  end 

aav@aav ~/p/p/withNone (master)> ponyc
Building builtin -> /usr/local/Cellar/ponyc/0.10.0/packages/builtin
Building . -> /Users/aav/pony-protobuf/pony/withNone
Error:
/Users/aav/pony-protobuf/pony/withNone/with_none.pony:8:5: try expression never results in an error
    with d = Disposable do
    ^

Environment:

aav@aav ~/p/p/withNone (master) [255]> ponyc --version
0.10.0 [release]
compiled with: llvm 3.9.0 -- Apple LLVM version 8.0.0 (clang-800.0.42.1)

[jemc]

In the sync call, we discussed that it should be possible to avoid adding None as one of the possible types of the result value. To do this, we'd probably want to make TK_WITH a first-class construct instead of sugaring it to a TK_TRY_NO_CHECK.

We also discussed that we want to look at changing the fact that with will silently swallow errors in the with block - we think that raising the errors up out of the block would be more usable/useful.

We also discussed that we want to take a broader look at how the try and with constructs work in the language, and re-evaluate the design more holistically. I personally was proposing something that turned out to be similar to OCaml's two types of try blocks, though they have apparently been the subject of some confusion on the part of users, so we want to be careful. Either way, this broader redesign would have to be part of an RFC.

[jemc]

Closing - not yet resolved. Conversation has moved to ticket #3759.