Allow indentation and braces (instead of ->) in match clauses

[osa1]

Currently handler and match are somewhat similar in that they specify which operator or value constructor they match, and on the right-hand side what to do with them, but the syntax for them are not exactly the same, which looks inconsistent.

Consider:

fun untry_(ex: either<string, a>): <exn_> a
  match ex
    Right(x) -> x
    Left(exn) -> throw_(exn)

val try_ = handler
  return(x) -> Right(x)
  ctl throw_(exn: string) -> Left(exn)

Here both handler and match use ->.

In handler, I can also indent the right-hand side and omit ->:

// also works
val try_ = handler
  return(x)
    Right(x)

  ctl throw_(exn: string)
    Left(exn)

But I can't do the same in match:

// syntax error
fun untry_(ex: either<string, a>): <exn_> a
  match ex
    Right(x)
      x

    Left(exn)
      throw_(exn)

Since indentation is just braces omitted, I can also use braces in handler if I want to define a handler in one line:

// also works
val try_ = handler
  return(x) { Right(x) }
  ctl throw_(exn: string) { Left(exn) }

Which also doesn't work for match.

I suggest we get rid of -> in match and use braces. That would make the syntax consistent with handler, and allow for cleaner match expressions when the right-hand sides are indented.

[TimWhiting]

Hmm, that's interesting. I didn't know that handlers could use (->).

I think the reason it is needed in match stems from the ability for match patterns to include guards.

For example:

match ex
  Right(x) | x.is-y && x.isnot-z -> println("Here")
  ...

The issue with allowing omitting -> is that any guards would need explicit braces to prevent confusion of guards and bodies. Do you start parsing the rhs as a guard or expression?

match ex
  Right(x) x.is-y && x.isnot-z println("Here")
  ...

One way we could get around this is to move the guard to the rhs of the guard like this

match ex
  Right(x) x.is-y && x.isnot-z | println("Here")
  ...

However for more complex matches where the match pattern is nontrivial you would really like a separator between the pattern and the guard itself - at least to assist parsing without as much lookahead maybe.

match (ex,ex2)
  (Right(x), Right(x2)) x.is-y && x.isnot-z | println("Here")
  ...

I assume the intent of braces is that for 99% of code the braces are optional or assist in readability. If braces where always required even for short match body expressions I personally think it would clutter the matches and reduce readability. For some advanced programmers braces might be preferred, but I don't think they help the learning curve of newer programmers, especially since 1) we have editors that do much better about highlighting & folding regions, and 2) having to learn everywhere you need to put braces before being able to write simple programs is mentally taxing to newer programmers.

[osa1]

I think the reason it is needed in match stems from the ability for match patterns to include guards.

Right, I think with guards there needs to be some separator between the guard expression and the right-hand side of the clause.

I initially though braces could be used as the separator

match ex
  Right(x) | x.is-y && x.isnot-z println("Here") // invalid

match ex
  Right(x) | x.is-y && x.isnot-z { println("Here") } // OK

but the braced expression will be parsed as a trailing lambda in the guard expression.

So maybe we can keep the | and -> in guards but allow braces and indentation without guards. However that will also be somewhat consistent in single line match clauses:

// With guards, we allow an expression after `->`, without indentation or braces
match ex
  Right(x) | x.is-y && x.isnot-z -> println("Here")

// Without guards, the expression need to be wrapped with braces, or indented:
match ex
  Right(x) println("Here") // syntax error

match ex
  Right(x) { println("Here") } // OK

match ex
  Right(x)
    println("Here") // OK