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Merge pull request #268 from alexflint/readme-custom-error-handling 

Add info to README about programmatically reproducing behavior of MustParse
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Alex Flint 2024-09-04 10:31:40 -04:00 committed by GitHub
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@ -105,7 +105,7 @@ $ NUM_WORKERS=4 ./example
Workers: 4 Workers: 4
``` ```
You can provide multiple values using the CSV (RFC 4180) format: You can provide multiple values in environment variables using commas:
```go ```go
var args struct { var args struct {
@ -120,7 +120,7 @@ $ WORKERS='1,99' ./example
Workers: [1 99] Workers: [1 99]
``` ```
You can also have an environment variable that doesn't match the arg name: Command line arguments take precedence over environment variables:
```go ```go
var args struct { var args struct {
@ -175,20 +175,7 @@ var args struct {
arg.MustParse(&args) arg.MustParse(&args)
``` ```
### Default values (before v1.2) Command line arguments take precedence over environment variables, which take precedence over default values. This means that we check whether a certain option was provided on the command line, then if not, we check for an environment variable (only if an `env` tag was provided), then if none is found, we check for a `default` tag containing a default value.
```go
var args struct {
Foo string
Bar bool
}
arg.Foo = "abc"
arg.MustParse(&args)
```
### Combining command line options, environment variables, and default values
You can combine command line arguments, environment variables, and default values. Command line arguments take precedence over environment variables, which take precedence over default values. This means that we check whether a certain option was provided on the command line, then if not, we check for an environment variable (only if an `env` tag was provided), then if none is found, we check for a `default` tag containing a default value.
```go ```go
var args struct { var args struct {
@ -198,10 +185,6 @@ arg.MustParse(&args)
``` ```
#### Ignoring environment variables and/or default values #### Ignoring environment variables and/or default values
The values in an existing structure can be kept in-tact by ignoring environment
variables and/or default values.
```go ```go
var args struct { var args struct {
Test string `arg:"-t,env:TEST" default:"something"` Test string `arg:"-t,env:TEST" default:"something"`
@ -261,26 +244,7 @@ fmt.Println(args.UserIDs)
map[john:123 mary:456] map[john:123 mary:456]
``` ```
### Custom validation
```go
var args struct {
Foo string
Bar string
}
p := arg.MustParse(&args)
if args.Foo == "" && args.Bar == "" {
p.Fail("you must provide either --foo or --bar")
}
```
```shell
./example
Usage: samples [--foo FOO] [--bar BAR]
error: you must provide either --foo or --bar
```
### Version strings ### Version strings
```go ```go
type args struct { type args struct {
... ...
@ -304,6 +268,24 @@ someprogram 4.3.0
> **Note** > **Note**
> If a `--version` flag is defined in `args` or any subcommand, it overrides the built-in versioning. > If a `--version` flag is defined in `args` or any subcommand, it overrides the built-in versioning.
### Custom validation
```go
var args struct {
Foo string
Bar string
}
p := arg.MustParse(&args)
if args.Foo == "" && args.Bar == "" {
p.Fail("you must provide either --foo or --bar")
}
```
```shell
./example
Usage: samples [--foo FOO] [--bar BAR]
error: you must provide either --foo or --bar
```
### Overriding option names ### Overriding option names
```go ```go
@ -452,8 +434,6 @@ main.NameDotName{Head:"file", Tail:"txt"}
### Custom placeholders ### Custom placeholders
*Introduced in version 1.3.0*
Use the `placeholder` tag to control which placeholder text is used in the usage text. Use the `placeholder` tag to control which placeholder text is used in the usage text.
```go ```go
@ -541,8 +521,6 @@ For more information visit github.com/alexflint/go-arg
### Subcommands ### Subcommands
*Introduced in version 1.1.0*
Subcommands are commonly used in tools that wish to group multiple functions into a single program. An example is the `git` tool: Subcommands are commonly used in tools that wish to group multiple functions into a single program. An example is the `git` tool:
```shell ```shell
$ git checkout [arguments specific to checking out code] $ git checkout [arguments specific to checking out code]
@ -603,6 +581,184 @@ if p.Subcommand() == nil {
} }
``` ```
### Programmatic error handling
The following reproduces the internal logic of `MustParse` for the simple case where
you are not using subcommands or --version. This allows you to respond
programatically to --help, and to any errors that come up.
```go
var args struct {
Something string
}
p, err := arg.NewParser(arg.Config{}, &args)
if err != nil {
log.Fatalf("there was an error in the definition of the Go struct: %v", err)
}
err = p.Parse(os.Args[1:])
switch {
case err == arg.ErrHelp: // indicates that user wrote "--help" on command line
p.WriteHelp(os.Stdout)
os.Exit(0)
case err != nil:
fmt.Printf("error: %v\n", err)
p.WriteUsage(os.Stdout)
os.Exit(1)
}
```
```shell
$ go run ./example --help
Usage: ./example --something SOMETHING
Options:
--something SOMETHING
--help, -h display this help and exit
$ ./example --wrong
error: unknown argument --wrong
Usage: ./example --something SOMETHING
$ ./example
error: --something is required
Usage: ./example --something SOMETHING
$ ./example --something abc
got "abc"
```
To also handle --version programatically, use the following:
```go
type args struct {
Something string
}
func (args) Version() string {
return "1.2.3"
}
func main() {
var args args
p, err := arg.NewParser(arg.Config{}, &args)
if err != nil {
log.Fatalf("there was an error in the definition of the Go struct: %v", err)
}
err = p.Parse(os.Args[1:])
switch {
case err == arg.ErrHelp: // found "--help" on command line
p.WriteHelp(os.Stdout)
os.Exit(0)
case err == arg.ErrVersion: // found "--version" on command line
fmt.Println(args.Version())
os.Exit(0)
case err != nil:
fmt.Printf("error: %v\n", err)
p.WriteUsage(os.Stdout)
os.Exit(1)
}
fmt.Printf("got %q\n", args.Something)
}
```
```shell
$ ./example --version
1.2.3
$ go run ./example --help
1.2.3
Usage: example --something SOMETHING
Options:
--something SOMETHING
--help, -h display this help and exit
$ ./example --wrong
1.2.3
error: unknown argument --wrong
Usage: example --something SOMETHING
$ ./example
error: --something is required
Usage: example --something SOMETHING
$ ./example --something abc
got "abc"
```
To also handle subcommands, use this most general version (also works in absence of subcommands but
is a bit more complex):
```go
type fetchCmd struct {
Count int
}
type args struct {
Something string
Fetch *fetchCmd `arg:"subcommand"`
}
func (args) Version() string {
return "1.2.3"
}
func main() {
var args args
p, err := arg.NewParser(arg.Config{}, &args)
if err != nil {
log.Fatalf("there was an error in the definition of the Go struct: %v", err)
}
err = p.Parse(os.Args[1:])
switch {
case err == arg.ErrHelp: // found "--help" on command line
p.WriteHelpForSubcommand(os.Stdout, p.SubcommandNames()...)
os.Exit(0)
case err == arg.ErrVersion: // found "--version" on command line
fmt.Println(args.Version())
os.Exit(0)
case err != nil:
fmt.Printf("error: %v\n", err)
p.WriteUsageForSubcommand(os.Stdout, p.SubcommandNames()...)
os.Exit(1)
}
}```
```shell
$ ./example --version
1.2.3
$ ./example --help
1.2.3
Usage: example [--something SOMETHING] <command> [<args>]
Options:
--something SOMETHING
--help, -h display this help and exit
--version display version and exit
Commands:
fetch
$ ./example fetch --help
1.2.3
Usage: example fetch [--count COUNT]
Options:
--count COUNT
Global options:
--something SOMETHING
--help, -h display this help and exit
--version display version and exit
```
### API Documentation ### API Documentation
https://godoc.org/github.com/alexflint/go-arg https://godoc.org/github.com/alexflint/go-arg
@ -619,4 +775,4 @@ The idea behind `go-arg` is that Go already has an excellent way to describe dat
### Backward compatibility notes ### Backward compatibility notes
Earlier versions of this library required the help text to be part of the `arg` tag. This is still supported but is now deprecated. Instead, you should use a separate `help` tag, described above, which removes most of the limits on the text you can write. In particular, you will need to use the new `help` tag if your help text includes any commas. Earlier versions of this library required the help text to be part of the `arg` tag. This is still supported but is now deprecated. Instead, you should use a separate `help` tag, described above, which makes it possible to include commas inside help text.