feat: errutil.Reduce (#18)

* test: add initial spec and tests for errutil.Reduce

* feat: implement errutil.Reduce

Implementation is not most efficient, naive recursive

* docs: improve Reduce docs

* feat: add initial Merge function

* feat: add nil filtering on merge function

* example: add merge example

* fix: remove non-sense comments

* fix: remove unnecessary return statement

* docs: improve docs

* feat: remove Merge and add nil filtering to reduce

* fix: allow reducer to return/receive nils

* docs: improve

* docs: improve docs

Makefile

@@ -1,10 +1,15 @@
+coverage=coverage.txt
+
 all: lint test bench
 
 test:
-	go test -race -timeout 10s -coverprofile=coverage.txt -covermode=atomic ./...
+	go test -race -timeout 10s -coverprofile=$(coverage) -covermode=atomic ./...
 
 test/%:
-	go test -race -timeout 10s -coverprofile=coverage.txt -covermode=atomic -run="${*}" ./...
+	go test -race -timeout 10s -coverprofile=$(coverage) -covermode=atomic -run="${*}" ./...
+
+coverage/show: test
+	go tool cover -html=$(coverage)
 
 fmt:
 	gofmt -s -w .

errutil/error.go

@@ -4,6 +4,11 @@
 // Utilities include:
 //
 // - An error type that makes it easy to work with const error sentinels.
+// - An easy way to wrap a list of errors together.
+// - An easy way to reduce a list of errors.
+//
+// Flexible enough that you can do your own wrapping/merging logic
+// but in a functional/simple way.
 package errutil
 
 import "errors"
@@ -14,6 +19,9 @@ import "errors"
 // as it's base type.
 type Error string
 
+// Reducer reduces 2 errors into one
+type Reducer func(error, error) error
+
 // Error return a string representation of the error.
 func (e Error) Error() string {
 	return string(e)
@@ -40,6 +48,24 @@ func Chain(errs ...error) error {
 	}
 }
 
+// Reduce will reduce all errors to a single one using the
+// provided reduce function.
+//
+// If errs is empty it returns nil, if errs has a single err
+// (len(errs) == 1) it will return the err itself.
+//
+// Nil errors on the errs args will be filtered out initially,
+// before reducing, so you can expect errors passed to the reducer
+// to be always non-nil.
+//
+// But if the reducer function itself returns nil, then the returned nil
+// won't be filtered and will be passed as an argument on the next
+// reducing step.
+func Reduce(r Reducer, errs ...error) error {
+	errs = removeNils(errs)
+	return reduce(r, errs...)
+}
+
 type errorChain struct {
 	head error
 	tail error
@@ -74,3 +100,15 @@ func removeNils(errs []error) []error {
 	}
 	return res
 }
+
+func reduce(r Reducer, errs ...error) error {
+	if len(errs) == 0 {
+		return nil
+	}
+	if len(errs) == 1 {
+		return errs[0]
+	}
+	err1, err2 := errs[0], errs[1]
+	err := r(err1, err2)
+	return reduce(r, append([]error{err}, errs[2:]...)...)
+}

errutil/error_example_test.go

@@ -39,9 +39,36 @@ func ExampleChain() {
 	fmt.Println(errors.Is(err, layer1Err))
 	fmt.Println(errors.Is(err, layer2Err))
 	fmt.Println(errors.Is(err, layer3Err))
+	fmt.Println(err)
 
 	// Output:
 	// true
 	// true
 	// true
+	// layer1Err: layer2Err: layer3Err
+}
+
+func ExampleReduce() {
+	// call multiple functions that may return an error
+	// but none of them should interrupt overall computation
+	var i int
+	someFunc := func() error {
+		i++
+		return fmt.Errorf("error %d", i)
+	}
+
+	var errs []error
+
+	errs = append(errs, someFunc())
+	errs = append(errs, someFunc())
+	errs = append(errs, someFunc())
+
+	err := errutil.Reduce(func(err1, err2 error) error {
+		return fmt.Errorf("%v,%v", err1, err2)
+	}, errs...)
+
+	fmt.Println(err)
+
+	// Output:
+	// error 1,error 2,error 3
 }

errutil/error_test.go

@@ -251,9 +251,228 @@ func TestErrorChainRespectIsMethodOfChainedErrors(t *testing.T) {
 	}
 }
 
-// To test the Is method the error must not be comparable.
+func TestErrorReducing(t *testing.T) {
+	type testcase struct {
+		name    string
+		errs    []error
+		reduce  errutil.Reducer
+		want    string
+		wantNil bool
+	}
+
+	mergeWithComma := func(err1, err2 error) error {
+		return fmt.Errorf("%v,%v", err1, err2)
+	}
+
+	tests := []testcase{
+		{
+			name: "reducing empty err list wont call reducer and returns nil",
+			errs: []error{},
+			reduce: func(err1, err2 error) error {
+				panic("unreachable")
+			},
+			wantNil: true,
+		},
+		{
+			name: "reducing one error wont call reducer and returns error",
+			errs: []error{errors.New("one")},
+			reduce: func(err1, err2 error) error {
+				panic("should not be called")
+			},
+			want: "one",
+		},
+		{
+			name:   "reducing two errors",
+			errs:   []error{errors.New("one"), errors.New("two")},
+			reduce: mergeWithComma,
+			want:   "one,two",
+		},
+		{
+			name: "reducing three errors",
+			errs: []error{
+				errors.New("one"),
+				errors.New("two"),
+				errors.New("three"),
+			},
+			reduce: mergeWithComma,
+			want:   "one,two,three",
+		},
+		{
+			name: "filtering just first err of 3",
+			errs: []error{
+				errors.New("one"),
+				errors.New("two"),
+				errors.New("three"),
+			},
+			reduce: func(err1, err2 error) error {
+				return err1
+			},
+			want: "one",
+		},
+		{
+			name: "filtering just first err of single err",
+			errs: []error{errors.New("one")},
+			reduce: func(err1, err2 error) error {
+				return err1
+			},
+			want: "one",
+		},
+		{
+			name: "filtering just second err",
+			errs: []error{
+				errors.New("one"),
+				errors.New("two"),
+				errors.New("three"),
+			},
+			reduce: func(err1, err2 error) error {
+				return err2
+			},
+			want: "three",
+		},
+		{
+			name: "reduces 3 errs to nil",
+			errs: []error{
+				errors.New("one"),
+				errors.New("two"),
+				errors.New("three"),
+			},
+			reduce: func(err1, err2 error) error {
+				return nil
+			},
+			wantNil: true,
+		},
+		{
+			name: "reduces 2 errs to nil",
+			errs: []error{
+				errors.New("one"),
+				errors.New("two"),
+			},
+			reduce: func(err1, err2 error) error {
+				return nil
+			},
+			wantNil: true,
+		},
+		{
+			name: "first is nil",
+			errs: []error{
+				nil,
+				errors.New("error 2"),
+				errors.New("error 3"),
+			},
+			reduce: mergeWithComma,
+			want:   "error 2,error 3",
+		},
+		{
+			name: "second is nil",
+			errs: []error{
+				errors.New("error 1"),
+				nil,
+				errors.New("error 3"),
+			},
+			reduce: mergeWithComma,
+			want:   "error 1,error 3",
+		},
+		{
+			name: "third is nil",
+			errs: []error{
+				errors.New("error 1"),
+				errors.New("error 2"),
+				nil,
+			},
+			reduce: mergeWithComma,
+			want:   "error 1,error 2",
+		},
+		{
+			name: "multiple nils interleaved",
+			errs: []error{
+				nil,
+				nil,
+				nil,
+				errors.New("error 1"),
+				nil,
+				nil,
+				errors.New("error 2"),
+				nil,
+				nil,
+			},
+			reduce: mergeWithComma,
+			want:   "error 1,error 2",
+		},
+		{
+			name: "first err among nils",
+			errs: []error{
+				errors.New("error 1"),
+				nil,
+				nil,
+				nil,
+			},
+			reduce: mergeWithComma,
+			want:   "error 1",
+		},
+		{
+			name: "last err among nils",
+			errs: []error{
+				nil,
+				nil,
+				nil,
+				errors.New("error 1"),
+			},
+			reduce: mergeWithComma,
+			want:   "error 1",
+		},
+		{
+			name: "reduces list with nils to nil",
+			errs: []error{
+				nil,
+				nil,
+				nil,
+			},
+			reduce:  mergeWithComma,
+			wantNil: true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			g := errutil.Reduce(test.reduce, test.errs...)
+
+			if test.wantNil {
+				if g == nil {
+					return
+				}
+				t.Fatalf(
+					"errutil.Reduce(%v)=%q; want nil",
+					test.errs,
+					g,
+				)
+			}
+
+			if g == nil {
+				t.Fatalf(
+					"errutil.Reduce(%v)=nil; want %q",
+					test.errs,
+					test.want,
+				)
+			}
+
+			got := g.Error()
+			want := test.want
+
+			if got != want {
+				t.Fatalf(
+					"errutil.Reduce(%v)=%q; want=%q",
+					test.errs,
+					got,
+					want,
+				)
+			}
+		})
+	}
+}
+
+// To test the Is method the error base type must not be comparable.
 // If it is comparable, Go always just compares it, the Is method
-// is just a fallback, not an override of actual behavior.
+// is just a fallback, not an override of actual comparison behavior.
 type errorThatNeverIs []string
 
 func (e errorThatNeverIs) Is(err error) bool {