csharplang/meetings/2017/LDM-2017-11-29.md

C# Language Design Notes for Nov 29, 2017

Warning: These are raw notes, and still need to be cleaned up. Read at your own peril!

Match expression

Current proposal:

• Infix vs prefix switch
• switch vs match
• curlies or parens for body
• case or not
• arrows?
• discards or defaults?
• also need to remember when clauses

If it's too similar to switch statements, then it sets certain expectations. If it's too different, it's not utilizing existing intuition.

Should it be a verb/command? Not many expression keywords are that (select is an exception, though).

case is heavyweight, but helps visually separate the issues.

            state = (state, action) switch (
                (DoorState.Closed, Action.Open) => DoorState.Opened,
                (DoorState.Opened, Action.Close) => DoorState.Closed,
                (DoorState.Closed, Action.Lock) => DoorState.Locked,
                (DoorState.Locked, Action.Unlock) => DoorState.Closed,
                _ => state);

            state = match (state, action) 
            {
                (DoorState.Closed, Action.Open) => DoorState.Opened,
                (DoorState.Opened, Action.Close) => DoorState.Closed,
                (DoorState.Closed, Action.Lock) => DoorState.Locked,
                (DoorState.Locked, Action.Unlock) => DoorState.Closed,
                _ => state
            };

            state = switch (state, action)
            {
                case (DoorState.Closed, Action.Open): DoorState.Opened
                case (DoorState.Opened, Action.Close): DoorState.Closed
                case (DoorState.Closed, Action.Lock): DoorState.Locked
                case (DoorState.Locked, Action.Unlock): DoorState.Closed
                case _: state
            };

The last one is subject to ambiguity-like situations between expression and statement switch.

We should also consider nesting of match expressions.

No matter what syntax we choose, we'll get requests for doing more things in expressions. We can live with that.

Parens look too much like a list of expressions.

Arrows make it look like lambda expressions.

Decisions

We agree that we will not use the keyword default. You can use _, and in the rare case where that's defined, you can use var _.

We like curly braces for the grouping.

The rest is up in the air. We'll stay with the first version for now in the prototype, other than the curly braces.

Where and when can identifier appear?

Syntax for property patterns

Not urgent