nix repl renders strings improperly escaped

[sternenseemann]

Describe the bug

nix repl prints string values improperly sometimes where pasting the printed expression back into the repl would result in a syntax error due to missing escaping.

Steps To Reproduce

Evaluate ''''${checkFlagsArray[@]}'' or "\${checkFlagsArray[@]}" into nix repl and paste the printed result back:

nix-repl> ''''${checkFlagsArray[@]}'' 
"${checkFlagsArray[@]}"

nix-repl> "\${checkFlagsArray[@]}"    
"${checkFlagsArray[@]}"

nix-repl> "${checkFlagsArray[@]}"
error: syntax error, unexpected '@'

       at «string»:1:20:

            1| "${checkFlagsArray[@]}"
             |                    ^
            2|

Expected behavior

nix repl prints "\${checkFlagsArray[@]}"

Additional context

Affects nixUnstable and nix 2.13.11.

[roberth]

This isn't rendering a string, but presenting a Nix language parsing error in a confusing way.
The wrong part is «string», which should have been something like «repl». Below that is your input, not the supposed string your input intends to represent.

Does this look less confusing?

nix-repl> "${checkFlagsArray[@]}"
error: syntax error, unexpected '@'

       at «repl»:1:20:

            1| "${checkFlagsArray[@]}"
             |                    ^
            2|

An even nicer error would catch this specific mistake and show a custom error message explaining what you probably did wrong. Something like: it seems like you're using bash interpolation syntax inside a Nix interpolated string. Did you forget to escape the $ sign?

[sternenseemann]

I don't really care about the error message, to be honest. The main issue is that the repl is actually rendering nix values in a way that they are not valid nix, i. e. a properly escaped nix string will evaluate to a nix string which is then rendered by the repl as invalid nix syntax:

nix-repl> ''''${checkFlagsArray[@]}'' 
"${checkFlagsArray[@]}"

The error message was just added to demonstrate that pasting back the printed nix expression in the repl will result in an error.

[stale]

I marked this as stale due to inactivity. → More info

[sternenseemann]

Hasn't been fixed.

[stale]

I marked this as stale due to inactivity. → More info

[sternenseemann]

Still an issue in Nix 2.9.

[sternenseemann]

Basically, $ is never escaped, I guess because Nix reuses escaping rules for C++ which doesn't have string interpolation:

nix-repl> "\${foo}"
"${foo}"

[sternenseemann]

Hm, this was fixed in #4012 for nix-instantiate at least:

$ nix-instantiate --eval -E '"\${foo}"'
"\${foo}"

I don't see why nix repl should behave differently here?

Edit:

nix/src/libcmd/repl.cc

Line 891 in 7c3ab57

// FIXME: lot of cut&paste from Nix's eval.cc.

[cyntheticfox]

Cool, I just ran into this today. I like to use REPLs though a lot, and might try to see about writing a PR to fix this. Might take a second though -- I'm a little out-of-practice with C++.

[roberth]

Hi @cyntheticfox, I've encountered the cause of the issue while resolving some tech debt.
I didn't read your message before, so I just wanted to let you know that a contribution would have been appreciated (but don't feel bad about it!).