Options for writable fs.FS extension (Create and Append)

[ben...@gmail.com]

Hi folks,

I'm considering (in draft PR https://github.com/benhoyt/goawk/pull/296) adding the option to GoAWK to allow customizing the file read/write operations that it performs when reading files, creating new files with AWK's ">" operator, or appending to them with AWK's ">>" operator.

I'd like it to be shaped like io/fs.FS. However, that only supports read-only files (Open returns an fs.File). So my thought is to have an interface extension like so:

type WriteFS interface {
fs.FS
Create(name string) (io.WriteCloser, error)
Append(name string) (io.WriteCloser, error)
}

Which are all the operations GoAWK needs.

Then if you pass a regular fs.FS (via Config.FileSystem) it'll only allow reads, but if you psas something that implements WriteFS as defined above, it'll support read and write.

I notice that some other libraries define WriteFS as:

type WriteFS interface {

    fs.FS
    OpenFile(name string, flag int, perm os.FileMode) (io.WriteCloser, error)

}

That fits nicely with os.OpenFile, but it pulls in a whole lot of POSIX-specific stuff and generality with the "flag" and "perm" arguments, when I only need Create and Append very specifically.

Does what I've proposed seem reasonable? Any better ideas for handling this?

-Ben

[Ian Lance Taylor]

On Mon, Jul 27, 2026 at 8:15 PM ben...@gmail.com <ben...@gmail.com> wrote:
>
> I'd like it to be shaped like io/fs.FS. However, that only supports read-only files (Open returns an fs.File). So my thought is to have an interface extension like so:
>
> type WriteFS interface {
> fs.FS
> Create(name string) (io.WriteCloser, error)
> Append(name string) (io.WriteCloser, error)
> }

There is some previous discussion at https://go.dev/issue/45757.

TL;DR: It's complicated.

Ian

[Ben Hoyt]

Thanks, Ian. That was an interesting thread -- yes, complicated! However, it seems that at least Create() was suggested several times with the same basic shape, so I guess I'm aiming in the right direction. My Append() seems to be novel, but I think "fits". It avoids the interface having to think about OS-level mode/perm/flags.

[Def Ceb]

Personally, I think Append() would be a bit too high level for a *filesystem* interface, and that something more like FUSE would make more sense. Maybe with some Capabilities() method to signal what functionality you actually implement.

It does get a bit too muddy the more I think about it though.

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[Mohamed Zane Attahri]

Feels like the addition of os.Root to stdlib somewhat standardized what a writable FS looks like. 

On Jul 28, 2026, at 4:24 AM, Def Ceb <mikk....@gmail.com> wrote:



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