Reason: `++` is now left-associative

Created on 12 Oct 2017 · 9Comments · Source: reasonml/reason

Previously, Reason and OCaml's ^ are right-associative. Now it's left-associative. A small consequence of this is that the old snippet:

a ^ b ^ c

now converts to

a ++ (b ++ c)

instead of

a ++ b ++ c

cc @let-def

Source

chenglou

Most helpful comment

If ++ is left-associative, it has no impact on string concatenation, but if it was overloaded for list concatenation, then a chain of ++ degenerates to a quadratic runtime, whereas a right-associative one is linear.
++ should be right-associativefor this reason :P .

let-def on 12 Oct 2017

👍4

All 9 comments

@chenglou If in 1.13.7, it was left-associative, then when converting shouldn't it convert to a ++ b ++ c?

1.13.7 would parse a ^ b ^ c as (a ^ b) ^ c, and then when passing to 3.0.0's printer, it would print as a ++ b ++ c right?

jordwalke on 12 Oct 2017

I got the left/right wrong. Fixed. The issue's still here though

chenglou on 12 Oct 2017

FWIW, it appears ^ was actually right associative in 1.13.7.

jordwalke on 12 Oct 2017

So I don't think there's anything wrong with this.
Except, you could make it so that ++ is also right associative in 3.0.0, so that when printing, there's no parens.
It might not matter for string concatenation because string concat is commutative.
It's worth seeing how it will play with |> and make the call based on that.

If this is easy to fix (make it right-associative) and doesn't cause other precedence issues, then let's do it. Otherwise it's no big deal and we can close this. Waiting for confirmation.

chenglou on 12 Oct 2017

Only downside is that + is left associative so it would be confusing if ++ was right associative. Maybe it doesn't matter since ++ will nearly always be used for string concat which we know abides by the associative property a ++ (b ++ c) is always equal to (a ++ b) ++ c. It would be rare for someone to be bitten by an assumption that ++ was left associative when it's really right associative. Debugging might be a bit unintuitive.

jordwalke on 12 Oct 2017

I'm inclined to vote right associative for ++.

jordwalke on 12 Oct 2017