strange string interpolation result

[kbkpbot]

Describe the bug

see code
may related to #26382

Reproduction Steps

i.v

module main

fn main() {
        mut x := '${'
        x += '}a'
        dump(x)
}

v run i.v

Expected Behavior

not sure , what is the expected result?

Current Behavior

$ v run i.v
[i.v:6] x:
        x += a

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.5.0 394abc1.09aa031

Environment details (OS name and version, etc.)

V full version	V 0.5.0 394abc1.09aa031
OS	linux, Ubuntu 24.04.3 LTS
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz
Memory	11.93GB/15.51GB
V executable	/media/HD/github/kbkpbot/v/v
V last modified time	2026-01-19 15:04:56
V home dir	OK, value: /media/HD/github/kbkpbot/v
VMODULES	OK, value: /home/mars/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/mars/v/bug/str
Git version	git version 2.43.0
V git status	weekly.2025.52-228-g09aa031b
.git/config present	true
cc version	cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
gcc version	gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
clang version	Ubuntu clang version 18.1.3 (1ubuntu1)
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.39-0ubuntu8.6) 2.39

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[JalonSolov]

I think expected result should be an error because there is no closing } in the declaration of x.

Doesn't matter that it's added on the next line. The definition of string interpolation is ${ ... some V stuff... }.

Would you want it to work like

mut str := '$'
str += '{'
str += '5'
str +' '}'

?

I vote that the full ${ v stuff } be treated as an "atomic" construct, meaning it must all be inside the same set of quotes.

[kbkpbot]

Another Example:

println('Run ${'v help'} for usage.')

This produces the output:

Run v help for usage.

While this works, it's confusing because:

1. The outer string uses single quotes
2. The inner interpolation also uses single quotes
3. This creates a nested quote pattern that shouldn't be allowed

Suggested Behavior:

The compiler should enforce that ${...} must be complete within the same string literal. If ${ is not
followed by a valid expression and a closing }, it should produce a compilation error.

This would make the language more predictable and prevent confusion about what constitutes valid string
interpolation syntax.

[JalonSolov]

Agreed, except on the quotes. Anything inside the ${ and } should be treated as if it was a separate V instance, as far as parsing, etc.

Otherwise,

println('He said "You'll be sorry!"')

would be invalid, while it works now (as it should).

[spytheman]

This would make the language more predictable and prevent confusion about what constitutes valid string interpolation syntax.

The language is already predictable for the parser - it expects ${ to be finalized/balanced by a } , and treats anything in between those, as a V expression (which allows for 'inner string' too, which can be confusing for the human reader).

I do not see a good way to make it more predictable, without also crippling it for valid use cases, like generated code for instance.

[spytheman]

What do you think of just changing vfmt, to produce:
'${ ' and "${ " for such situations, thus making them easier to recognize for human readers?

[spytheman]

See also #24198 which has this example:

import term

fn main() {
	println('Hey it\'s ${term.blue('Tony\'s')} car !')
}