nasin/config/config.go

// Package config provides a configuration system for applications.
package config

import "fmt"
import "math"
import "strconv"
import "git.tebibyte.media/tomo/tomo/event"

type configError string;
func (err configError) Error () string { return string(err) }

const (
	// ErrClosed is returned when Get/Set/Reset is called after the config
	// has been closed.
	ErrClosed = configError("attempt to access a closed config")
	// ErrNonexistentEntry is returned when an entry was not found.
	ErrNonexistentEntry = configError("nonexistent entry")
	// ErrMalformedEntry is returned when a config entry could not be
	// parsed.
	ErrMalformedEntry = configError("malformed entry")
	// ErrMalformedKey is returned when a key has invalid runes.
	ErrMalformedKey = configError("malformed key")
	// ErrMalformedValue is returned when a value could not be parsed.
	ErrMalformedValue = configError("malformed value")
	// ErrMalformedString is returned when a string value could not be
	// parsed.
	ErrMalformedStringValue = configError("malformed string value")
	// ErrMalformedNumber is returned when a number value could not be
	// parsed.
	ErrMalformedNumberValue = configError("malformed number value")
	// ErrMalformedBool is returned when a boolean value could not be
	// parsed.
	ErrMalformedBoolValue = configError("malformed bool value")
	// ErrMalformedEscapeSequence us returned when an escape sequence could
	// not be parsed.
	ErrMalformedEscapeSequence = configError("malformed escape sequence")
)

// Config provides access to an application's configuration, and can notify an
// application of changes to it.
type Config interface {
	// Get gets a value, first considering the user-level config file, and
	// then falling back to system level config files. If the value could
	// not be found anywhere, the specified fallback value is returned. If
	// the key is invalid, it returns nil, ErrMalformedKey.
	Get (key string, fallback Value) (Value, error)
	// GetString is like Get, but will only return strings. If the value is
	// not a string, it will return fallback.
	GetString (key string, fallback string) (string, error)
	// GetNumber is like Get, but will only return numbers. If the value is
	// not a number, it will return fallback.
	GetNumber (key string, fallback float64) (float64, error)
	// GetBool is like Get, but will only return booleans. If the value is
	// not a boolean, it will return fallback.
	GetBool (key string, fallback bool) (bool, error)
	// Set sets a value in the user-level config file. If the key is
	// invalid, it returns ErrMalformedKey.
	Set (key string, value Value) error
	// Reset removes the value from the user-level config file, resetting it
	// to what is described by the system-level config files. If the key is
	// invalid, it returns ErrMalformedKey.
	Reset (key string) error
	// OnChange specifies a function to be called whenever a value is
	// changed. The callback is always run within the backend's event loop
	// using tomo.Do. This could have been a channel but I didn't want to do
	// that to people.
	OnChange (func (key string)) event.Cookie
}

// ConfigCloser is a config with a Close behavior, which stops watching the
// config file and causes any subsequent sets/gets to return errors. Anything
// that receives a ConfigCloser must close it when done.
type ConfigCloser interface {
	Config
	// Close closes the config, causing it to stop watching for changes.
	// Reads or writes to the config after this will return an error.
	Close () error
}

var negativeZero = math.Copysign(0, -1)

// Value is a config value. Its String behavior produces a lossless and
// syntactically valid representation of the value.
type Value interface {
	value ()
	fmt.Stringer
}

// ValueString is a string value.
type ValueString string
var _ Value = ValueString("")
func (ValueString) value () { }
func (value ValueString) String () string {
	return fmt.Sprintf("\"%s\"", escape(string(value)))
}

// ValueNumber is a number value.
type ValueNumber float64
var _ Value = ValueNumber(0)
func (ValueNumber) value () { }
func (value ValueNumber) String () string {
	number := float64(value)
	// the requirements I wrote said lossless in all cases. here's lossless
	// in all cases!
	switch {
	case math.IsInf(number, 0):
		if math.Signbit(number) {
			return "-Inf"
		} else {
			return "Inf"
		}
		
	case math.IsNaN(number):
		return "NaN"
		
	case number == 0, number == negativeZero:
		if math.Signbit(number) {
			return "-0"
		} else {
			return "0"
		}
		
	case math.Round(number) == number:
		return strconv.FormatInt(int64(number), 10)
		
	default:
		return strconv.FormatFloat(number, 'f', -1, 64)
	}
}


// ValueBool is a boolean value.
var _ Value = ValueBool(false)
type ValueBool bool
func (ValueBool) value () { }
func (value ValueBool) String () string {
	if value {
		return "true"
	} else {
		return "false"
	}
}