types: add Float64() to currency

types/currency.go

@@ -12,6 +12,8 @@ import (
 )
 
 var (
+	scPrecisionBig = new(big.Int).Exp(big.NewInt(10), big.NewInt(-24), nil)
+
 	// ZeroCurrency represents zero base units.
 	ZeroCurrency Currency
 
@@ -276,6 +278,12 @@ func (c Currency) String() string {
 	return s
 }
 
+// Float64 returns the value of c as a float64 and a bool indicating whether f
+// represents x exactly.
+func (c Currency) Float64() (float64, bool) {
+	return new(big.Rat).SetFrac(c.Big(), scPrecisionBig).Float64()
+}
+
 // Format implements fmt.Formatter. It accepts the following formats:
 //
 //	d: raw integer (equivalent to ExactString())