add tests for ff

pkg/ff/compose.go

@@ -6,18 +6,3 @@ func Compose[T1, T2, T3 any](fn1 func(T1) T2, fn2 func(T2) T3) func(T1) T3 {
 		return fn2(fn1(x))
 	}
 }
-
-// FIXME: signarure about to be changed
-func ComposeErr[T1, T2, T3 any](fn1 func(T1) (T2, error), fn2 func(T2) (T3, error), errHandler func(error)) func(T1) T3 {
-	return func(x T1) T3 {
-		res, err := fn1(x)
-		if err != nil {
-			errHandler(err)
-		}
-		res2, err := fn2(res)
-		if err != nil {
-			errHandler(err)
-		}
-		return res2
-	}
-}

pkg/ff/ff_test.go

@@ -0,0 +1,55 @@
+package ff
+
+import (
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCompose(t *testing.T) {
+	fn1 := func(x int) int {
+		return x + 1
+	}
+
+	fn2 := func(x int) string {
+		return strconv.Itoa(x)
+	}
+
+	composedFn := Compose(fn1, fn2)
+
+	result := composedFn(5)
+	require.Equal(t, "6", result, "Unexpected result for Compose")
+
+	result = composedFn(10)
+	require.Equal(t, "11", result, "Unexpected result for Compose")
+}
+
+func TestMap(t *testing.T) {
+	a := []int{1, 2, 3, 4, 5}
+
+	fn := func(x int) string {
+		return strconv.Itoa(x)
+	}
+
+	piper := Map(a, fn).Do()
+
+	// Iterate through the values and test the output
+	expected := []string{"1", "2", "3", "4", "5"}
+	for i, val := range piper {
+		require.Equal(t, expected[i], val, "Unexpected result for Map")
+	}
+}
+
+func TestReduce(t *testing.T) {
+	a := []int{1, 2, 3, 4, 5}
+
+	sum := func(result *int, x *int) int {
+		return *result + *x
+	}
+
+	result := Reduce(a, sum)
+
+	expected := 15
+	require.Equal(t, expected, result, "Unexpected result for Reduce")
+}

pkg/pipe/prefixpipe.go

@@ -62,7 +62,7 @@ func Reduce[SrcT any, DstT any](p Piper[SrcT], fn func(*DstT, *SrcT) DstT, initV
 	default:
 		res := fn(&init, &data[0])
 		for i := range data[1:] {
-			res = fn(&res, &data[i])
+			res = fn(&res, &data[i+1])
 		}
 		return res
 	}