fixed panic when using mutators on floats

README.md

@@ -204,6 +204,26 @@ day: 2 | month: 1 | year: 2006
 day: 2 | month: 2 | year: 1962
 ```
 
+### Adding 2 values together
+
+Input:
+```
+5 7
+3 2
+10 152
+-15 3.7
+```
+
+Command: `format -i '(-?\d+) (-?\d+(?:.\d+)?)' -o '{1} + {2} = {1:%.2f:+(2)}'`
+
+Output:
+```
+5 + 7 = 12.00
+3 + 2 = 5.00
+10 + 152 = 162.00
+-15 + 3.7 = -11.30
+```
+
 ### Bulk renaming files
 
 Rename a bunch of files using format at once

main.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"reflect"
 	"regexp"
 	"strconv"
 	"strings"
@@ -164,14 +165,13 @@ func numMut2func[T int64 | float64](mutation string) (f func(value T, vars []str
 		mutators = append(mutators, mut)
 	}
 
+	numberParser := number_parser[T]()
+
 	return func(value T, vars []string) T {
 		for _, mutator := range mutators {
 			var otherValue T
 			if mutator.Var {
-				other, err := strconv.Atoi(vars[mutator.Value])
-				if err != nil {
-					panic("back reference group for number mutator is not a number")
-				}
+				other := numberParser(vars[mutator.Value])
 				otherValue = T(other)
 			} else {
 				otherValue = T(mutator.Value)
@@ -193,3 +193,29 @@ func numMut2func[T int64 | float64](mutation string) (f func(value T, vars []str
 		return value
 	}
 }
+
+func number_parser[T int64 | float64]() func(str string) T {
+	typeOfT := reflect.TypeOf(new(T)).Elem()
+	typeOfInt64 := reflect.TypeOf(new(int64)).Elem()
+	typeOfFloat64 := reflect.TypeOf(new(float64)).Elem()
+
+	if typeOfT == typeOfInt64 {
+		return func(str string) T {
+			num, err := strconv.Atoi(str)
+			if err != nil {
+				panic("expected integer but found " + str)
+			}
+			return T(num)
+		}
+	} else if typeOfT == typeOfFloat64 {
+		return func(str string) T {
+			num, err := strconv.ParseFloat(str, 64)
+			if err != nil {
+				panic("expected float but found " + str)
+			}
+			return T(num)
+		}
+	}
+
+	panic("invalid number type")
+}