Generics and Custom Iterators

[mecolosimo]

V version: V 0.4.9 9f3f129, press to see full `v doctor` output

V full version	V 0.4.9 2684ef9.9f3f129
OS	linux, Linux Mint 22
Processor	8 cpus, 64bit, little endian, AMD Ryzen 5 7520U with Radeon Graphics
Memory	0.66GB/14.86GB
V executable	/home/mcolosimo/github/v/v
V last modified time	2025-03-13 16:39:39
V home dir	OK, value: /home/mcolosimo/github/v
VMODULES	OK, value: /home/mcolosimo/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/mcolosimo/SyncProjects/mecolosimo/generics
Git version	git version 2.43.0
V git status	weekly.2025.11-25-g9f3f1291
.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	N/A
tcc version	tcc version 0.9.28rc 2024-07-31 HEAD@1cee0908 (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 0134e9b9
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.39-0ubuntu8.4) 2.39

What did you do?
./v -g -o vdbg cmd/v && ./vdbg src/main.v && src/main

module main

struct SquareIterator {
    arr []int
mut:
    idx int
}

fn (mut iter SquareIterator) next[K]() ?K {
    if iter.idx >= iter.arr.len {
        return none
    }
    defer {
        iter.idx++
    }
    return iter.arr[iter.idx] * iter.arr[iter.idx]
}

fn main() {
	nums := [1, 2, 3, 4, 5]
	iter := SquareIterator{
		arr: nums
	}

	for squared in iter {
		println(squared)
	}
}

What did you see?

/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/markused/walker.v:602: at v__markused__Walker_fn_decl: RUNTIME ERROR: invalid memory access
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/markused/walker.v:216: by v__markused__Walker_stmt
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/markused/walker.v:279: by v__markused__Walker_stmts
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/markused/walker.v:614: by v__markused__Walker_fn_decl
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/markused/walker.v:125: by v__markused__Walker_mark_root_fns
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/markused/markused.v:477: by v__markused__mark_used
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/builder/builder.v:154: by v__builder__Builder_middle_stages
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/builder/builder.v:163: by v__builder__Builder_front_and_middle_stages
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/builder/cbuilder/cbuilder.v:72: by v__builder__cbuilder__gen_c
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/builder/cbuilder/cbuilder.v:60: by v__builder__cbuilder__build_c
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/builder/cbuilder/cbuilder.v:50: by v__builder__cbuilder__compile_c
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/builder/rebuilding.v:325: by v__builder__Builder_rebuild
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/vlib/v/builder/compile.v:17: by v__builder__compile
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/cmd/v/v.v:192: by main__rebuild
/tmp/v_1000/../../../../../../home/mcolosimo/github/v/cmd/v/v.v:150: by main__main
/tmp/v_1000/../../../../../../tmp/v_1000/vdbg.01JP86431QD8JBGYTR2GNG1AAK.tmp.c:39624: by main

What did you expect to see?

It compiles

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[huly-for-github]

Connected to Huly®: V_0.6-22325

[mecolosimo]

Following the docs on (generics)[https://docs.vlang.io/type-declarations.html#generics]...

module main

struct SquareIterator[K] {
    arr []int
mut:
    idx int
}

fn (mut iter SquareIterator[K]) next() ?K {
    if iter.idx >= iter.arr.len {
        return none
    }
    defer {
        iter.idx++
    }
    return iter.arr[iter.idx] * iter.arr[iter.idx]
}

fn main() {
	nums := [1, 2, 3, 4, 5]
	iter := SquareIterator[int]{
		arr: nums
	}

	for squared in iter {
		println(squared)
	}
}

This simple example now works (compiles and runs).

[Delta456]

Why even have generic template if none of the struct members use it?

[jorgeluismireles]

If you were use interpolation in printing like this:

	for squared in iter {
		println('${squared}')
	}

you would had a hint that the problem is related with the generic, since above change causes this error:

error: no known default format for type `K`
   22 |     }
   23 |     for squared in iter {
   24 |         println('${squared}')
      |                    ~~~~~~~

Removing the unecessary generic makes the program work:

fn (mut iter SquareIterator) next() ?int {

But is a fact that using println(squared) instead the interpolation, the compiler does not indicate the generic problem.

[Delta456]

I can't seem to reproduce this on the playground nor locally. Check the code here: https://play.vlang.io/p/2491cd02a8

[mecolosimo]

@Delta456 you used the "correct" version. Use fn (mut iter SquareIterator) next[K]() ?K {. This was me trying out stuff to see how to do it (which was wrong).

[Delta456]

I get

code.v:9:14: error: generic struct `SquareIterator` in fn declaration must specify the generic type names, e.g. SquareIterator[T]
    7 | }
    8 | 
    9 | fn (mut iter SquareIterator) next[K]() ?K {
      |              ~~~~~~~~~~~~~~
   10 |     if iter.idx >= iter.arr.len {
   11 |         return none

[jorgeluismireles]

By doing arr []K we could use it for other than integers:

struct SquareIterator[K] {
    arr []K
mut:
    idx int
}

fn (mut iter SquareIterator[K]) next() ?K {
    if iter.idx >= iter.arr.len {
        return none
    }
    defer {
        iter.idx++
    }
    return iter.arr[iter.idx] * iter.arr[iter.idx]
}

fn main() {
	nums := [1.0, 2, 3, 4, 5]
	iter := SquareIterator[f64]{
		arr: nums
	}

	for squared in iter {
		 println('${squared}')
	}
}

[Delta456]

Yes. I assume the issue is solved and can be closed now?

[mecolosimo]

The original submitted code does work still grant it was wrong but it should have bulked at that.

[JalonSolov]

The original code still throws a C error, so there is still something to fix, here. V should catch whatever is wrong before it ever gets to the C compiler, even if there is a workaround.

Attempting to run the code with just v -g run, shows the following:

/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:45212: at v__markused__Walker_fn_decl: RUNTIME ERROR: invalid memory access
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:44773: by v__markused__Walker_stmt
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:44847: by v__markused__Walker_stmts
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:45224: by v__markused__Walker_fn_decl
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:44665: by v__markused__Walker_mark_root_fns
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:44334: by v__markused__mark_used
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:49450: by v__builder__Builder_middle_stages
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:49466: by v__builder__Builder_front_and_middle_stages
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:51977: by v__builder__cbuilder__gen_c
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:51949: by v__builder__cbuilder__build_c
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:51940: by v__builder__cbuilder__compile_c
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:51857: by v__builder__Builder_rebuild
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:51010: by v__builder__compile
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:52369: by main__rebuild
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:52285: by main__main
/tmp/v_1000/v2.01JP8ADZZDAJBK6852119JW18P.tmp.c:53044: by main

Attempting to run it with v -g -no-skip-unused run only shows the following:

cgen error: could not generate string method `K_str` for type `K`

Therefore, the problem is definitely in V, with the default -skip-unused.

[spytheman]

I've fixed the crash in 8c92e3d . Now the default is the same as with -no-skip-unused:

#0 07:03:35 ^ master ~/code/v>v -g -no-skip-unused run x.v
cgen error: could not generate string method `K_str` for type `K`
#1 07:05:51 ^ master ~/code/v>v -g run x.v
cgen error: could not generate string method `K_str` for type `K`
#1 07:05:53 ^ master ~/code/v>

We should definitely improve the error message could not generate string method though - it should be a proper checker error with source position etc, not a cgen one, that lacks context...