text/template: provide a way to trim leading and trailing space between actions

Borrowing a suggestion from the issue listed below, we modify the lexer to
trim spaces at the beginning (end) of a block of text if the action immediately
before (after) is marked with a minus sign. To avoid parsing/lexing ambiguity,
we require an ASCII space between the minus sign and the rest of the action.
Thus:

	{{23 -}}
	<
	{{- 45}}

produces the output
	23<45

All the work is done in the lexer. The modification is invisible to the parser
or any outside package (except I guess for noticing some gaps in the input
if one tracks error positions). Thus it slips in without worry in text/template
and html/template both.

Fixes long-requested issue #9969.

Change-Id: I3774be650bfa6370cb993d0899aa669c211de7b2
Reviewed-on: https://go-review.googlesource.com/14391
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/text/template/doc.go

@@ -36,6 +36,31 @@ Here is a trivial example that prints "17 items are made of wool".
 
 More intricate examples appear below.
 
+Text and spaces
+
+By default, all text between actions is copied verbatim when the template is
+executed. For example, the string " items are made of " in the example above appears
+on standard output when the program is run.
+
+However, to aid in formatting template source code, if an action's left delimiter
+(by default "{{") is followed immediately by a minus sign and ASCII space character
+("{{- "), all trailing white space is trimmed from the immediately preceding text.
+Similarly, if the right delimiter ("}}") is preceded by a space and minus sign
+(" -}}"), all leading white space is trimmed from the immediately following text.
+In these trim markers, the ASCII space must be present; "{{-3}}" parses as an
+action containing the number -3.
+
+For instance, when executing the template whose source is
+
+	"{{23 -}} < {{- 45}}"
+
+the generated output would be
+
+	"23<45"
+
+For this trimming, the definition of white space characters is the same as in Go:
+space, horizontal tab, carriage return, and newline.
+
 Actions
 
 Here is the list of actions. "Arguments" and "pipelines" are evaluations of

src/text/template/example_test.go

@@ -15,9 +15,12 @@ func ExampleTemplate() {
 	const letter = `
 Dear {{.Name}},
 {{if .Attended}}
-It was a pleasure to see you at the wedding.{{else}}
-It is a shame you couldn't make it to the wedding.{{end}}
-{{with .Gift}}Thank you for the lovely {{.}}.
+It was a pleasure to see you at the wedding.
+{{- else}}
+It is a shame you couldn't make it to the wedding.
+{{- end}}
+{{with .Gift -}}
+Thank you for the lovely {{.}}.
 {{end}}
 Best wishes,
 Josie

src/text/template/exec_test.go

@@ -797,18 +797,19 @@ type Tree struct {
 }
 
 // Use different delimiters to test Set.Delims.
+// Also test the trimming of leading and trailing spaces.
 const treeTemplate = `
-	(define "tree")
+	(- define "tree" -)
 	[
-		(.Val)
-		(with .Left)
-			(template "tree" .)
-		(end)
-		(with .Right)
-			(template "tree" .)
-		(end)
+		(- .Val -)
+		(- with .Left -)
+			(template "tree" . -)
+		(- end -)
+		(- with .Right -)
+			(- template "tree" . -)
+		(- end -)
 	]
-	(end)
+	(- end -)
 `
 
 func TestTree(t *testing.T) {
@@ -853,19 +854,13 @@ func TestTree(t *testing.T) {
 		t.Fatal("parse error:", err)
 	}
 	var b bytes.Buffer
-	stripSpace := func(r rune) rune {
-		if r == '\t' || r == '\n' {
-			return -1
-		}
-		return r
-	}
 	const expect = "[1[2[3[4]][5[6]]][7[8[9]][10[11]]]]"
 	// First by looking up the template.
 	err = tmpl.Lookup("tree").Execute(&b, tree)
 	if err != nil {
 		t.Fatal("exec error:", err)
 	}
-	result := strings.Map(stripSpace, b.String())
+	result := b.String()
 	if result != expect {
 		t.Errorf("expected %q got %q", expect, result)
 	}
@@ -875,7 +870,7 @@ func TestTree(t *testing.T) {
 	if err != nil {
 		t.Fatal("exec error:", err)
 	}
-	result = strings.Map(stripSpace, b.String())
+	result = b.String()
 	if result != expect {
 		t.Errorf("expected %q got %q", expect, result)
 	}

src/text/template/parse/lex.go

@@ -83,6 +83,21 @@ var key = map[string]itemType{
 
 const eof = -1
 
+// Trimming spaces.
+// If the action begins "{{- " rather than "{{", then all space/tab/newlines
+// preceding the action are trimmed; conversely if it ends " -}}" the
+// leading spaces are trimmed. This is done entirely in the lexer; the
+// parser never sees it happen. We require an ASCII space to be
+// present to avoid ambiguity with things like "{{-3}}". It reads
+// better with the space present anyway. For simplicity, only ASCII
+// space does the job.
+const (
+	spaceChars      = " \t\r\n" // These are the space characters defined by Go itself.
+	leftTrimMarker  = "- "      // Attached to left delimiter, trims trailing spaces from preceding text.
+	rightTrimMarker = " -"      // Attached to right delimiter, trims leading spaces from following text.
+	trimMarkerLen   = Pos(len(leftTrimMarker))
+)
+
 // stateFn represents the state of the scanner as a function that returns the next state.
 type stateFn func(*lexer) stateFn
 
@@ -220,10 +235,18 @@ const (
 // lexText scans until an opening action delimiter, "{{".
 func lexText(l *lexer) stateFn {
 	for {
-		if strings.HasPrefix(l.input[l.pos:], l.leftDelim) {
+		delim, trimSpace := l.atLeftDelim()
+		if delim {
+			trimLength := Pos(0)
+			if trimSpace {
+				trimLength = rightTrimLength(l.input[l.start:l.pos])
+			}
+			l.pos -= trimLength
 			if l.pos > l.start {
 				l.emit(itemText)
 			}
+			l.pos += trimLength
+			l.ignore()
 			return lexLeftDelim
 		}
 		if l.next() == eof {
@@ -238,13 +261,56 @@ func lexText(l *lexer) stateFn {
 	return nil
 }
 
-// lexLeftDelim scans the left delimiter, which is known to be present.
+// atLeftDelim reports whether the lexer is at a left delimiter, possibly followed by a trim marker.
+func (l *lexer) atLeftDelim() (delim, trimSpaces bool) {
+	if !strings.HasPrefix(l.input[l.pos:], l.leftDelim) {
+		return false, false
+	}
+	// The left delim might have the marker afterwards.
+	trimSpaces = strings.HasPrefix(l.input[l.pos+Pos(len(l.leftDelim)):], leftTrimMarker)
+	return true, trimSpaces
+}
+
+// rightTrimLength returns the length of the spaces at the end of the string.
+func rightTrimLength(s string) Pos {
+	return Pos(len(s) - len(strings.TrimRight(s, spaceChars)))
+}
+
+// atRightDelim reports whether the lexer is at a right delimiter, possibly preceded by a trim marker.
+func (l *lexer) atRightDelim() (delim, trimSpaces bool) {
+	if strings.HasPrefix(l.input[l.pos:], l.rightDelim) {
+		return true, false
+	}
+	// The right delim might have the marker before.
+	if strings.HasPrefix(l.input[l.pos:], rightTrimMarker) {
+		if strings.HasPrefix(l.input[l.pos+trimMarkerLen:], l.rightDelim) {
+			return true, true
+		}
+	}
+	return false, false
+}
+
+// leftTrimLength returns the length of the spaces at the beginning of the string.
+func leftTrimLength(s string) Pos {
+	return Pos(len(s) - len(strings.TrimLeft(s, spaceChars)))
+}
+
+// lexLeftDelim scans the left delimiter, which is known to be present, possibly with a trim marker.
 func lexLeftDelim(l *lexer) stateFn {
 	l.pos += Pos(len(l.leftDelim))
-	if strings.HasPrefix(l.input[l.pos:], leftComment) {
+	trimSpace := strings.HasPrefix(l.input[l.pos:], leftTrimMarker)
+	afterMarker := Pos(0)
+	if trimSpace {
+		afterMarker = trimMarkerLen
+	}
+	if strings.HasPrefix(l.input[l.pos+afterMarker:], leftComment) {
+		l.pos += afterMarker
+		l.ignore()
 		return lexComment
 	}
 	l.emit(itemLeftDelim)
+	l.pos += afterMarker
+	l.ignore()
 	l.parenDepth = 0
 	return lexInsideAction
 }
@@ -257,19 +323,34 @@ func lexComment(l *lexer) stateFn {
 		return l.errorf("unclosed comment")
 	}
 	l.pos += Pos(i + len(rightComment))
-	if !strings.HasPrefix(l.input[l.pos:], l.rightDelim) {
+	delim, trimSpace := l.atRightDelim()
+	if !delim {
 		return l.errorf("comment ends before closing delimiter")
-
+	}
+	if trimSpace {
+		l.pos += trimMarkerLen
 	}
 	l.pos += Pos(len(l.rightDelim))
+	if trimSpace {
+		l.pos += leftTrimLength(l.input[l.pos:])
+	}
 	l.ignore()
 	return lexText
 }
 
-// lexRightDelim scans the right delimiter, which is known to be present.
+// lexRightDelim scans the right delimiter, which is known to be present, possibly with a trim marker.
 func lexRightDelim(l *lexer) stateFn {
+	trimSpace := strings.HasPrefix(l.input[l.pos:], rightTrimMarker)
+	if trimSpace {
+		l.pos += trimMarkerLen
+		l.ignore()
+	}
 	l.pos += Pos(len(l.rightDelim))
 	l.emit(itemRightDelim)
+	if trimSpace {
+		l.pos += leftTrimLength(l.input[l.pos:])
+		l.ignore()
+	}
 	return lexText
 }
 
@@ -278,7 +359,8 @@ func lexInsideAction(l *lexer) stateFn {
 	// Either number, quoted string, or identifier.
 	// Spaces separate arguments; runs of spaces turn into itemSpace.
 	// Pipe symbols separate and are emitted.
-	if strings.HasPrefix(l.input[l.pos:], l.rightDelim) {
+	delim, _ := l.atRightDelim()
+	if delim {
 		if l.parenDepth == 0 {
 			return lexRightDelim
 		}

src/text/template/parse/lex_test.go

@@ -278,6 +278,19 @@ var lexTests = []lexTest{
 		tRight,
 		tEOF,
 	}},
+	{"trimming spaces before and after", "hello- {{- 3 -}} -world", []item{
+		{itemText, 0, "hello-"},
+		tLeft,
+		{itemNumber, 0, "3"},
+		tRight,
+		{itemText, 0, "-world"},
+		tEOF,
+	}},
+	{"trimming spaces before and after comment", "hello- {{- /* hello */ -}} -world", []item{
+		{itemText, 0, "hello-"},
+		{itemText, 0, "-world"},
+		tEOF,
+	}},
 	// errors
 	{"badchar", "#{{\x01}}", []item{
 		{itemText, 0, "#"},
@@ -339,7 +352,7 @@ var lexTests = []lexTest{
 		{itemText, 0, "hello-"},
 		{itemError, 0, `unclosed comment`},
 	}},
-	{"text with comment close separted from delim", "hello-{{/* */ }}-world", []item{
+	{"text with comment close separated from delim", "hello-{{/* */ }}-world", []item{
 		{itemText, 0, "hello-"},
 		{itemError, 0, `comment ends before closing delimiter`},
 	}},

src/text/template/parse/parse_test.go

@@ -228,6 +228,13 @@ var parseTests = []parseTest{
 		`{{with .X}}"hello"{{end}}`},
 	{"with with else", "{{with .X}}hello{{else}}goodbye{{end}}", noError,
 		`{{with .X}}"hello"{{else}}"goodbye"{{end}}`},
+	// Trimming spaces.
+	{"trim left", "x \r\n\t{{- 3}}", noError, `"x"{{3}}`},
+	{"trim right", "{{3 -}}\n\n\ty", noError, `{{3}}"y"`},
+	{"trim left and right", "x \r\n\t{{- 3 -}}\n\n\ty", noError, `"x"{{3}}"y"`},
+	{"comment trim left", "x \r\n\t{{- /* hi */}}", noError, `"x"`},
+	{"comment trim right", "{{/* hi */ -}}\n\n\ty", noError, `"y"`},
+	{"comment trim left and right", "x \r\n\t{{- /* */ -}}\n\n\ty", noError, `"x""y"`},
 	// Errors.
 	{"unclosed action", "hello{{range", hasError, ""},
 	{"unmatched end", "{{end}}", hasError, ""},