An Go example to read a rational number from yaml.

main.go

package main

import (
	"log"
	"math/big"
	"os"

	"gopkg.in/yaml.v2"

	"github.com/pkg/errors"
)

func main() {
	exampleYaml := `---
unit_price: 2.1
`
	var config ExampleConfig
	err := yaml.Unmarshal([]byte(exampleYaml), &config)
	if err != nil {
		log.Printf("%+v", err)
		os.Exit(1)
	}
	log.Printf("%+v", config)
}

type ExampleConfig struct {
	UnitPrice Rational `yaml:"unit_price"`
}

type Rational big.Rat

func (o *Rational) UnmarshalYAML(unmarshal func(v interface{}) error) error {
	var s string
	err := unmarshal(&s)
	if err != nil {
		return err
	}
	_, ok := (*big.Rat)(o).SetString(s)
	if !ok {
		return errors.Errorf("failed to convert to a rational number: %s", s)
	}
	return nil
}