I've noticed differing behavior if I specify a flag via the command line vs if that flag comes from a config file.

Here is an example:

package main

import (
    "errors"
    "flag"
    "os"

    "github.com/peterbourgon/ff"
)

func main() {
    fs := flag.NewFlagSet("my-program", flag.ExitOnError)
    var (
        _ = fs.String("config", "", "config file (optional)")
    )

    fs.Func("aflag", "a flag that raises a error",
        func(flagValue string) error {
            return errors.New("bad flag")
        },
    )

    ff.Parse(fs, os.Args[1:],
        ff.WithEnvVarPrefix("MY_PROGRAM"),
        ff.WithConfigFileFlag("config"),
        ff.WithConfigFileParser(ff.PlainParser),
    )
}

If I run it by passing the flag explicitly, I get

./my-program -aflag bad
invalid value "bad" for flag -aflag: bad flag
Usage of my-program:
-aflag value
a flag that raises a error
-config string
config file (optional)

but running ./my-program -config config.txt where config.txt is aflag bad executes without error. I know fs.Func runs and the error is returned in both cases, but I am not sure why it doesn't trigger an an exit in the latter case.

Thanks!