must

must offers a handful of functions for dealing with invariants, i.e. things that must be true, or else the program should panic.

Fallible Functions

Fallible functions are functions that may return an error. If an error is not recoverable, the proper response may be to panic. If so, you can get the value and panic on any error with the must.Get function:

response := must.Get(http.Get(url))

You may need a lower-order function to be consumed by a higher-order one, but can’t because the function you want returns an error along with a value. must.Of converts a function that returns a value and an error into one that only returns a value:

symbols := fluent.SliceOfStrings([]string{"1", "2"})
integers := symbols.ToInt(must.Of(strconv.Atoi))

err != nil checking is verbose. If the correct response to an error is to panic, then must.BeNil is the abbreviated version of panicking on an error:

err := response.Write(writer)
must.BeNil(err)

Helpers

There is currently one helper function that gives a panic-on-error version of a standard library function:

home := must.Getenv("HOME") // panic if HOME is unset or empty

Patterns

These patterns demonstrate idiomatic usage drawn from production code.

Initialization Sequences

Use must.Get and must.BeNil together for init/setup code where errors are fatal:

func OpenDatabase(cfg Config) *Database {
    db := must.Get(sql.Open("postgres", cfg.DSN))
    must.BeNil(db.Ping())

    db.SetMaxIdleConns(cfg.IdleConns)
    db.SetMaxOpenConns(cfg.OpenConns)

    return &Database{db: db}
}

This is cleaner than nested error checks when failure means the program cannot continue.

Config Loading

Combine with config parsing where missing config is a fatal error:

func LoadConfig() Config {
    var config Config
    must.BeNil(viper.ReadInConfig())
    must.BeNil(viper.Unmarshal(&config))
    return config
}

Embedding Files

Use with embed.FS when embedded files must exist:

//go:embed README.md
var files embed.FS

func GetReadme() string {
    return string(must.Get(files.ReadFile("README.md")))
}