internal/diff: fix printing and top-level diffs

Print concrete values: before %-v was used, which would
print conjuncts, not evaluated expressions.

Top-level diffs were awkward, as it would not return an edit
script and the user would have to do their own printing.
This fixes that.

TODO: modify the API to not return the kind, only the
EditScript.

Change-Id: I7c9d0322fb5e6dac0f4668f14de8ae76bb614823
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9342
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

internal/diff/diff.go

@@ -34,6 +34,8 @@ var (
 	}
 )
 
+// TODO: don't return Kind, which is always Modified or not.
+
 // Diff is a shorthand for Schema.Diff.
 func Diff(x, y cue.Value) (Kind, *EditScript) {
 	return Schema.Diff(x, y)
@@ -42,7 +44,11 @@ func Diff(x, y cue.Value) (Kind, *EditScript) {
 // Diff returns an edit script representing the difference between x and y.
 func (p *Profile) Diff(x, y cue.Value) (Kind, *EditScript) {
 	d := differ{cfg: *p}
-	return d.diffValue(x, y)
+	k, es := d.diffValue(x, y)
+	if k == Modified && es == nil {
+		es = &EditScript{x: x, y: y}
+	}
+	return k, es
 }
 
 // Kind identifies the kind of operation of an edit script.

internal/diff/diff_test.go

@@ -61,6 +61,9 @@ func TestDiff(t *testing.T) {
 		x:    `"foo"`,
 		y:    `"bar"`,
 		kind: Modified,
+		diff: `- "foo",
++ "bar",
+`,
 	}, {
 		name: "basics",
 		x: `{
@@ -111,7 +114,7 @@ func TestDiff(t *testing.T) {
 		`,
 		kind: Modified,
 		diff: `  {
-      ls: [2, 3, 4]
+      ls: [2,3,4]
 -     "foo-bar": 2
 +     "foo-bar": 3
       s: 4
@@ -125,7 +128,7 @@ func TestDiff(t *testing.T) {
       lm2: [
 -         6,
       ]
-+     la: [2, 3, 4]
++     la: [2,3,4]
   }
 `,
 	}, {
@@ -299,9 +302,7 @@ a: x: "hello"
 		`,
 		kind: Modified,
 		diff: `  {
--     a: {
--     	x: "hello"
--     }
+-     a: {x:"hello"}
   }
 `,
 	}, {

internal/diff/print.go

@@ -98,7 +98,8 @@ func (p *printer) script(e *EditScript) {
 	case cue.ListKind:
 		p.printList(e)
 	default:
-		p.printf("BadExpr")
+		p.printElem("-", e.x)
+		p.printElem("+", e.y)
 	}
 }
 
@@ -194,7 +195,7 @@ func (p *printer) printSkipped(n int) {
 
 func (p *printer) printValue(v cue.Value) {
 	// TODO: have indent option.
-	s := fmt.Sprintf("%-v", v)
+	s := fmt.Sprintf("%+v", v)
 	io.WriteString(p, s)
 }