# Extend Arg with Arg.File?
**URL:** https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999
**Category:** Learning
**Created:** [June 18, 2020, 9:55am UTC](https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999 "2020-06-18T09:55:30Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@dromas](https://discuss.ocaml.org/u/dromas)
#### Post date: [June 18, 2020, 9:55am UTC](https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999/1 "2020-06-18T09:55:30Z")
Hello,
is it possible to extend Arg with Arg.File?
### My use case
My (imaginary) tool has two options: `-o` and `-v`.
`-o` expects an output filename
`-v` turns verbose logging on.
So this is the spec:
```
[
"-o",
Arg.String (fun fname -> outfile := fname),
"output filename";
"-v",
Arg.Set verbose,
"verbose mode";
]
```
### Problem
When a user mistakenly omits the filename after `-o` and invokes the tool with `mytool -o -v` the output is written into a file “-v”.
Most likely, this wasn’t the intended behavior.
### My current workarround
Whenever a string argument is supposed to be a filename I make sure it dosn’t start with a dash:
```
"-o",
Arg.String (fun fname ->
match fname with
| "" ->
eprintf "-o: filename cannot be empty string";
exit 1
| "-" ->
outfile := fname (* stdin or stdout *)
| s when s.[0] = '-' ->
eprintf "-o: filename missing before %S\n" fname;
exit 1
| _ ->
outfile := fname
),
"outfile filename";
```
### Problem with my workaround
I need this over and over again and would like to handle it with something like
```
"-o",
Arg.File (fun fname -> outfile := fname),
"output filename";
```
which implements the workaround internally.
## My attempted solution
```
module Arg = struct
include Arg
type spec += File of (string -> unit)
end
```
### Problem with my attempted solution
It doesn’t compile :-/
```
Error: Type definition spec is not extensible
```
### Questions
- Is there a better way to handle the situation than the work around described above?
- Whould it be appropriate to add `Arg.File` to the Stdlib?
---
### Author:  [@nojb](https://discuss.ocaml.org/u/nojb)
#### Post date: [June 18, 2020, 10:13am UTC](https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999/2 "2020-06-18T10:13:41Z")
> [@dromas](#):
>
> ### Problem
>
> When a user mistakenly omits the filename after `-o` and invokes the tool with `mytool -o -v` the output is written into a file “-v”.
> Most likely, this wasn’t the intended behavior.
Note that this is the standard command-line behaviour in the Unix world (try it with your favourite C compiler).
> [@dromas](#):
>
> ### Problem with my attempted solution
>
> It doesn’t compile :-/
>
> ```auto
> Error: Type definition spec is not extensible
>
> ```
Only extensible sum types can be, well, extended. Ordinary sum types cannot be extended. Note that it wouldn’t make sense to extend `Arg.spec`, as the module `Arg` would not know what to do with the new constructor.
Best wishes,
Nicolás
---
### Author:  [@dromas](https://discuss.ocaml.org/u/dromas)
#### Post date: [June 18, 2020, 10:54am UTC](https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999/3 "2020-06-18T10:54:58Z")
> Note that this is the standard command-line behaviour in the Unix world (try it with your favourite C compiler).
You’re right.
I thought filenames with leading dashes must be “escaped” with `--` in
the Unix world, e.g. `gcc -o -- -v file.c`. Turns out this is not the
case and thus shouldn’t be “fixed” in the Stdlib.
> Only extensible sum types can be, well, extended.
> Ordinary sum types cannot be extended.
I skimed over [https://sites.google.com/site/ocamlopen/](https://sites.google.com/site/ocamlopen/) and
misinterpreted the “…” as arbitrary constructors.
Thus I concluded all types are extensible since 4.02.
I was wrong again.
> Note that it wouldn’t make sense to extend `Arg.spec`, as the module `Arg` would not know what to do with the new constructor.
Yes, this was just the first step.
---
### Author:  [@rbardou](https://discuss.ocaml.org/u/rbardou)
#### Post date: [June 18, 2020, 3:23pm UTC](https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999/4 "2020-06-18T15:23:32Z")
Instead of trying to extend the type, you can define a function to share your code:
```auto
let arg_file file_ref =
Arg.String (fun fname ->
match fname with
| "" ->
eprintf "-o: filename cannot be empty string";
exit 1
| "-" ->
file_ref := fname (* stdin or stdout *)
| s when s.[0] = '-' ->
eprintf "-o: filename missing before %S\n" fname;
exit 1
| _ ->
file_ref := fname
)
in
Arg.spec [
"-i", arg_file infile, " input file";
"-o", arg_file outfile, " output file";
]
```
Although that would prevent you from passing filenames which start with a dash, so I would not do that.
---
### Author:  [@Chet\_Murthy](https://discuss.ocaml.org/u/Chet_Murthy)
#### Post date: [June 18, 2020, 3:27pm UTC](https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999/5 "2020-06-18T15:27:06Z")
This isn’t really a solution, but … have you thought about learning to use the excellent, excellent (did I mention it’s excellent) `cmdliner` package? It was sort of meant for this sort of thing – adding behaviour to the processing of args in a modular way, I mean.
---
### Author:  [@dromas](https://discuss.ocaml.org/u/dromas)
#### Post date: [June 19, 2020, 8:32am UTC](https://discuss.ocaml.org/t/extend-arg-with-arg-file/5999/6 "2020-06-19T08:32:12Z")
> [@rbardou](#):
>
> Instead of trying to extend the type, you can define a function to share your code:
Thanks, this is viable, to0.
> [@rbardou](#):
>
> Although that would prevent you from passing filenames which start with a dash, so I would not do that.
This should still be possible with `--`.
> [@Chet\_Murthy](#):
>
> have you thought about learning to use the excellent, excellent (did I mention it’s excellent) `cmdliner` package?
Not really - thanks for the suggestion. I usually try to avoid non-stdlib packages if the functionality takes just a couple of lines.