`guard`-`else` about civet HOT 4 CLOSED

Pipe conditions do seem like a reasonable possible approach. We might need one for truthy and one for nonnull. We've talked about ?|> before to act like "if nonnull, then pipe into rhs".

Given how much guard resembles unless, I'd be tempted to spell it like so:

unless x? := getX()
  throw new Error "Missing x coordinate"
unless y? := getY() then return
unless z? := getZ() then return
setPoint x, y, z

#756 points out that our current meaning of unless x := ... isn't very helpful; it just exposes a falsey value for x inside the block. The proposed alternative would be to expose the declaration outside the unless block instead — essentially opposite of if, which kind of makes sense?

Alternatively, we could pull the declaration outside:

x := getX() ?? throw new Error "Missing x coordinate"
y := getY() ?? return
z := getZ() ?? return
setPoint x, y, z

This works for throw, but not for return, because we don't yet have a way to expressionize return. When it's at the top level of a declaration like this, I imagine we could handle it though.

from civet.

Supersedes #756 I suppose

from civet.

A couple thoughts:

I'm reluctant to add any more reserved words like guard. Would unless x? := getX() else return be an acceptable spelling of this feature?

Maybe upgrading pipes to better handle piping to return/throw and allowing something similar to declaration conditions could help with this. #897 (comment)

x := getX() !> return
---
let ref = getX(); if(!ref) return ref; const x = ref;

Possible conditional pipe options: ?>, !>, ?|>, !|> maybe something else?

from civet.

unless-else reads like a double-negative, but otherwise I'd be fine with it.

from civet.