templates: make "join" work with non-string slices and map values

Add a custom join function that allows for non-string slices to be
joined, following the same rules as "fmt.Sprint", it will use the
fmt.Stringer interface if implemented, or "error" if the type has
an "Error()".

For maps, it joins the map-values, for example:

    docker image inspect --format '{{join .Config.Labels ", "}}' ubuntu
    24.04, ubuntu

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

templates/templates.go

@@ -6,6 +6,9 @@ package templates
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
+	"reflect"
+	"sort"
 	"strings"
 	"text/template"
 )
@@ -26,7 +29,7 @@ var basicFunctions = template.FuncMap{
 		return strings.TrimSpace(buf.String())
 	},
 	"split":    strings.Split,
-	"join":     strings.Join,
+	"join":     joinElements,
 	"title":    strings.Title, //nolint:nolintlint,staticcheck // strings.Title is deprecated, but we only use it for ASCII, so replacing with golang.org/x/text is out of scope
 	"lower":    strings.ToLower,
 	"upper":    strings.ToUpper,
@@ -103,3 +106,40 @@ func truncateWithLength(source string, length int) string {
 	}
 	return source[:length]
 }
+
+// joinElements joins a slice of items with the given separator. It uses
+// [strings.Join] if it's a slice of strings, otherwise uses [fmt.Sprint]
+// to join each item to the output.
+func joinElements(elems any, sep string) (string, error) {
+	if elems == nil {
+		return "", nil
+	}
+
+	if ss, ok := elems.([]string); ok {
+		return strings.Join(ss, sep), nil
+	}
+
+	switch rv := reflect.ValueOf(elems); rv.Kind() {
+	case reflect.Array, reflect.Slice:
+		var b strings.Builder
+		for i := range rv.Len() {
+			if i > 0 {
+				b.WriteString(sep)
+			}
+			_, _ = fmt.Fprint(&b, rv.Index(i).Interface())
+		}
+		return b.String(), nil
+
+	case reflect.Map:
+		var out []string
+		for _, k := range rv.MapKeys() {
+			out = append(out, fmt.Sprint(rv.MapIndex(k).Interface()))
+		}
+		// Not ideal, but trying to keep a consistent order
+		sort.Strings(out)
+		return strings.Join(out, sep), nil
+
+	default:
+		return "", fmt.Errorf("expected slice, got %T", elems)
+	}
+}

templates/templates_test.go

@@ -3,6 +3,7 @@ package templates
 import (
 	"bytes"
 	"testing"
+	"text/template"
 
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
@@ -139,3 +140,87 @@ func TestHeaderFunctions(t *testing.T) {
 		})
 	}
 }
+
+type stringerString string
+
+func (s stringerString) String() string {
+	return "stringer" + string(s)
+}
+
+type stringerAndError string
+
+func (s stringerAndError) String() string {
+	return "stringer" + string(s)
+}
+
+func (s stringerAndError) Error() string {
+	return "error" + string(s)
+}
+
+func TestJoinElements(t *testing.T) {
+	tests := []struct {
+		doc    string
+		data   any
+		expOut string
+		expErr string
+	}{
+		{
+			doc:    "nil",
+			data:   nil,
+			expOut: `output: ""`,
+		},
+		{
+			doc:    "non-slice",
+			data:   "hello",
+			expOut: `output: "`,
+			expErr: `template: my-template:1:13: executing "my-template" at <join . ", ">: error calling join: expected slice, got string`,
+		},
+		{
+			doc:    "structs",
+			data:   []struct{ A, B string }{{"1", "2"}, {"3", "4"}},
+			expOut: `output: "{1 2}, {3 4}"`,
+		},
+		{
+			doc:    "map with strings",
+			data:   map[string]string{"A": "1", "B": "2", "C": "3"},
+			expOut: `output: "1, 2, 3"`,
+		},
+		{
+			doc:    "map with stringers",
+			data:   map[string]stringerString{"A": "1", "B": "2", "C": "3"},
+			expOut: `output: "stringer1, stringer2, stringer3"`,
+		},
+		{
+			doc:    "map with errors",
+			data:   []stringerAndError{"1", "2", "3"},
+			expOut: `output: "error1, error2, error3"`,
+		},
+		{
+			doc:    "stringers",
+			data:   []stringerString{"1", "2", "3"},
+			expOut: `output: "stringer1, stringer2, stringer3"`,
+		},
+		{
+			doc:    "stringer with errors",
+			data:   []stringerAndError{"1", "2", "3"},
+			expOut: `output: "error1, error2, error3"`,
+		},
+	}
+
+	const formatStr = `output: "{{- join . ", " -}}"`
+	tmpl, err := New("my-template").Funcs(template.FuncMap{"join": joinElements}).Parse(formatStr)
+	assert.NilError(t, err)
+
+	for _, tc := range tests {
+		t.Run(tc.doc, func(t *testing.T) {
+			var b bytes.Buffer
+			err := tmpl.Execute(&b, tc.data)
+			if tc.expErr != "" {
+				assert.ErrorContains(t, err, tc.expErr)
+			} else {
+				assert.NilError(t, err)
+			}
+			assert.Equal(t, b.String(), tc.expOut)
+		})
+	}
+}