rdmd --eval and --loop prints last @statement

[timotheecour]

/cc @wilzbach @andralex @marler8997 @WebDrake

TLDR

if no trailing ;, print last statment starting with @

rdmd --eval="@1+2"
3
rdmd --eval="auto a = 10; auto b = [a, a + 1]; @b"
[10, 11]

this PR avoids some of the disadvantages of https://github.com/dlang/tools/pull/317/files
I was using this idea for a repl i was working on and seemed useful.

Advantages:

• no breaking behavior [1]
• no guesswork of how to rewrite the last expression (eg previous PR choked on if(0) {} 2 + 2)
• no writeln unless user shows explicit intent (addresses rdmd: and with a last statement without effect should print it #317 (comment))

proposed syntax

use a delimiter (eg @ but let's not bikeshed about which one to use too much) to split the statment that shall be writeln'd:

NOTES

• not sure what's the best symbol, @ is a good choice as easy to type and unlikely to appear by accident
• TODO: could use something more robust based on tokenization to avoid pulling the wrong @ from:

rdmd --eval='"@"foo@bar"'

workaround: in this case the user can just use:

rdmd --eval='"writeln("foo@bar");'

and things will work. So tokenization could happen in future PR to improve things later

[1] no breaking behavior: what changes is that stuff that didnt' compile could now compile; that's acceptable tradeoff as it gives a meaning to previously invalid input

[dlang-bot]

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[joseph-wakeling-sociomantic]

The goal here has nice aspects, but what's not nice is changing --eval's behaviour so that in magic special cases it does something different from evaluating normal D code. It feels quite unnatural for --eval to have behavioural differences from normal D evaluation, and even more so for --eval to allow special magic syntax that's not allowed in normal D code.

How do you feel about the idea of instead implementing a new flag (--write ... ?) that is specifically dedicated to printing out the result of the provided statement?

[joseph-wakeling-sociomantic]

Slightly bigger picture concern: how do you decide what kind of statement is valid for these output purposes?

rdmd --write='"Hello, World!"'

... is obvious, but what about

rdmd --write='return'

... ?

[marler8997]

FWIW, I slightly preferred #317, but I don't have a strong opinion either way.