Vim9: cannot use key from dictionary as iteration variable

[lacygoill]

Steps to reproduce

Run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    def Func()
        var d = {k: 'aaa'}
        for d.k in ['bbb', 'ccc', 'ddd']
            echo d.k
        endfor
    enddef
    Func()
EOF
)

These lines are echo'ed:

aaa
aaa
aaa

Expected behavior

Not sure. Either these lines are echo'ed:

bbb
ccc
ddd

Or an error is given:

E1234: cannot use key from dictionary as iteration variable

In any case, I think the current situation is not OK, because it leads to unexpected results, which might be hard to debug.

Operating system

Ubuntu 20.04.3 LTS

Version of Vim

8.2 Included patches: 1-3640

Additional Context

No issue in legacy:

vim9script
function Func()
    let d = {'k': 'aaa'}
    for d.k in ['bbb', 'ccc', 'ddd']
        echo d.k
    endfor
endfunction
Func()

bbb
ccc
ddd

Are we meant to be able to use a dictionary key as iteration variable in Vim9? If not, maybe it should be documented.

[bfrg]

Some languages allow using a dictionary key for iteration. For example, in Python:

#!/usr/bin/env python3

d = {"k": "aaa"}

print(d["k"])

for d["k"] in ["bbb", "ccc", "ddd"]:
    print(d["k"])

print(d["k"])

Output:

aaa
bbb
ccc
ddd
ddd

[lacygoill]

Same behavior when we iterate with a list member:

vim9script
def Func()
    var l = ['aaa']
    for l[0] in ['bbb', 'ccc', 'ddd']
        echo l[0]
    endfor
enddef
Func()

aaa
aaa
aaa

No error is given, and l[0] silently fails to iterate over the members of ['bbb', 'ccc', 'ddd'].

And again, no issue in legacy:

vim9script
function Func()
    let l = ['aaa']
    for l[0] in ['bbb', 'ccc', 'ddd']
        echo l[0]
    endfor
endfunction
Func()

bbb
ccc
ddd

[lacygoill]

Just to be clear: if this syntax is deemed too confusing, that's fine. What is not OK is that Vim silently ignores the iterated list.

[brammool]

It works in legacy script, it should also work in Vim9 script

[brammool]

Let's first look at script level. Currently this gives an error: var i = 0 for i in [1, 2, 3] echo i endfor The reason is that it may accidentally use variable "i". It's like declaring a variable that was already declared. Now this also means that using a list or dict member gives the error for the variable already existing: var l = [0] for l[0] in [1, 2, 3] echo l[0] endfor If we would allow this, because it's a member and not a declared variable, things get confusing. So perhaps re-using a variable should not be an error? That might lead to mistakes. To be consistent, we should just say that the loop variable must not exist yet. That disallows assigning to a list or dict member, but is that really a problem? In case you need it there is just one extra line: var l = [0] for i in [1, 2, 3] l[0] = i echo l[0] endfor It's a small price to pay for simplicity. Extending this to compiled functions, it should work the same way. Then the problem is that this does not give an error: def Func() var l = [0] for l[0] in [1, 2, 3] echomsg l[0] endfor enddef I'll add the missing error check. Then it should work consistently.

…

-- BRIDGEKEEPER: What is your favorite colour? GAWAIN: Blue ... No yelloooooww! "Monty Python and the Holy Grail" PYTHON (MONTY) PICTURES LTD /// Bram Moolenaar -- ***@***.*** -- http://www.Moolenaar.net \\\ /// \\\ \\\ sponsor Vim, vote for features -- http://www.Vim.org/sponsor/ /// \\\ help me help AIDS victims -- http://ICCF-Holland.org ///