proposal syntax else ... then for else keyword in conditional branches

[lyrachord]

examples in tutorial

if a == b then
  env.out.print("they are the same")
else
  if a > b then
    env.out.print("a is bigger")
  else
    env.out.print("b bigger")
  end
end

limited by set pattern in old school

if a == b then
  env.out.print("they are the same")
elseif a > b then
  env.out.print("a is bigger")
else
  env.out.print("b bigger")
end

just use "else ... then" instead of "elseif ... then"

if a == b then
  env.out.print("they are the same")
else a > b then
  env.out.print("a is bigger")
else
  env.out.print("b bigger")
end

And it is ok in expressions

x = 1 + if lots then 100 else afew then 10 else 2 end

And there is almost no additional cost for parsing.

[SeanTAllen]

Without heavy justification in an RFC, I would be against this proposal. I would advise anyone writing an RFC for this to spend a good deal of time explaining why this is better than what currently exists as this adds nothing semantically that isn't already in the language.

[jemc]

What's the value/improvement that this would bring?

[pmetras]

What's the value/improvement that this would bring?

Removing a keyword elseif for a simpler language?

[jasoncarr0]

While simple is better, I don't know that everyone would agree that removing the keyword elseif would decrease complexity, rather than increase it.

[chalcolith]

I am not in favour of this change; I think that else ... then would be more confusing than elseif.

[jemc]

Even if we were to agree it is both simpler and better in absolute terms, it needs to provide enough value to justify overcoming the friction of a widely-breaking change.

If the value it provides is marginal or just notional, but not significant in the sense of providing meaningful/substantial value to users of Pony, then it's not worth the breaking change.

[rhagenson]

Dropping in to say I am also not in favor of this. Also here to add that ifelse would have to be removed from ifdef ... ifelse... else ... end for it to be fully removed.

[jemc]

@SeanTAllen also mentioned in today's sync that the parser is currently written to require no lookahead, and this would potentially break that current invariant.

[jemc]

@jasoncarr0 also mentioned that human parsers also have the same problem with lookahead 😉

[pmetras]

Though not clearly documented in the tutorial, one can simulate this syntax with pattern matching using guards, like:

actor Main
  new create(env: Env) =>
    let x: USize = 123
    match x
    | if x < 50 => env.out.print("< 50")
    | if x < 100 => env.out.print("< 100")
    | if x < 200 => env.out.print("< 200")
    else env.out.print("= 100")
    end