expr/docs/Configuration.md

Configuration

Expr can be configured to do more things. For example, with AllowUndefinedVariables or AsBool to expect the boolean result from the expression.

program, err := expr.Compile(code, expr.Env(Env{}), expr.AllowUndefinedVariables(), expr.AsBool())

Functions

Expr supports any Go functions. For example, you can use fmt.Sprintf or methods of your structs. Also, Expr supports functions configured via expr.Function(name, fn[, ...types]) option.

	atoi := expr.Function(
		"atoi",
		func(params ...any) (any, error) {
			return strconv.Atoi(params[0].(string))
		},
	)

	program, err := expr.Compile(`atoi("42")`, atoi)

Expr sees the atoi function as a function with a variadic number of arguments of type any and returns a value of type any. But, we can specify the types of arguments and the return value by adding the correct function signature or multiple signatures.

	atoi := expr.Function(
		"atoi",
		func(params ...any) (any, error) {
			return strconv.Atoi(params[0].(string))
		},
		new(func(string) int),
	)

Or we can simply reuse the strconv.Atoi function.

	atoi := expr.Function(
		"atoi",
		func(params ...any) (any, error) {
			return strconv.Atoi(params[0].(string))
		},
		strconv.Atoi,
	)

Here is another example with a few function signatures:

	toInt := expr.Function(
		"toInt",
		func(params ...any) (any, error) {
			switch params[0].(type) {
			case float64:
				return int(params[0].(float64)), nil
			case string:
				return strconv.Atoi(params[0].(string))
			}
			return nil, fmt.Errorf("invalid type")
		},
		new(func(float64) int),
		new(func(string) int),
	)