Add configuration comparison capability contract

docs/roadmap/implementation.md

@@ -124,10 +124,20 @@ ordinary acquisition for a comparison-only scope.
    when applicable to a comparison-capable scope. Rely on existing core binding
    validation for collisions with generated flags.
 
-**Completion evidence:** focused installation/discovery checks for comparison-only,
-mixed, unsupported, and metadata-only scopes; missing callback and retained nil
-loader issues; captured metadata surviving source edits. Execution is wired in
-Stages 3, 4, and 8; do not add temporary handlers for intermediate completeness.
+**Completion evidence — complete:** `Diff`, `ComparisonSide[I]`,
+`ComparisonRequest[I]`, and the optional `Scope.Compare` callback are declared in
+ckconfig. Capability validation now requires `Acquire` only for ordinary
+operations and `Compare` only for `Diff`; retained adapter issues still reach
+installation after capability narrowing. Operation normalization and discovery
+preserve the four existing operation order and append `diff`, generating the
+family command only when a scope declares comparison. Focused checks cover
+comparison-only, mixed, unsupported, and metadata-only scopes; missing provider
+wiring; a narrowed ConfigKit adapter retaining its nil-loader issue; and captured
+capability metadata and provider references surviving edits to the source
+declaration. Construction, installation, discovery, and unsupported-scope
+preflight leave factories and providers uninvoked. The ckconfig and dependent
+ckp suites pass. Input flags and successful comparison execution remain for
+their later owners.
 
 ## Stage 3: Add comparison flags, inheritance, and availability preflight
 

integrations/ckconfig/comparison_contract_test.go

@@ -0,0 +1,105 @@
+package ckconfig
+
+import (
+	"context"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+
+	commandkit "gitea.maximumdirect.net/eric/CommandKit"
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestComparisonCapabilitiesAreIndependentAndCaptured(t *testing.T) {
+	type input struct{ Value string }
+	var inputCalls, stateCalls, acquireCalls, compareCalls int
+	compare := func(context.Context, ComparisonRequest[input], struct{}) (commandkit.Acquired[[]configkit.Difference], error) {
+		compareCalls++
+		return commandkit.Acquired[[]configkit.Difference]{}, nil
+	}
+	operations := []Operation{Diff}
+	suite := Suite[input, struct{}]{
+		NewInput: func() input { inputCalls++; return input{} },
+		NewState: func() struct{} { stateCalls++; return struct{}{} },
+		Scopes: []Scope[input, struct{}]{
+			{Name: "app", Operations: operations, Compare: compare},
+			{Name: "promptkit", Operations: []Operation{Diff, Show, Validate}, Acquire: func(context.Context, Request, input, struct{}) (commandkit.Acquired[View], error) {
+				acquireCalls++
+				return commandkit.Acquired[View]{}, nil
+			}, Compare: compare},
+			{Name: "alpha"},
+			{Name: "plain", Operations: []Operation{Validate}, Acquire: func(context.Context, Request, input, struct{}) (commandkit.Acquired[View], error) {
+				acquireCalls++
+				return commandkit.Acquired[View]{}, nil
+			}},
+		},
+	}
+	captured := captureSuite(suite)
+	declaration := Command(suite)
+
+	// Capability metadata and callback references are copied during capture.
+	suite.Scopes[0].Operations[0] = Show
+	suite.Scopes[0].Compare = nil
+
+	app := newConfigTestApp(t, declaration)
+	var output strings.Builder
+	outcome := app.Execute(context.Background(), []string{"config", "scopes"}, commandkit.Streams{Out: &output, Err: io.Discard})
+	if outcome.Kind != commandkit.OutcomeSuccess {
+		t.Fatalf("scope discovery outcome = %#v", outcome)
+	}
+	if got, want := output.String(), "app: diff\npromptkit: validate, show, diff\nalpha: \nplain: validate\n"; got != want {
+		t.Fatalf("scope discovery = %q, want %q", got, want)
+	}
+
+	// A family-wide diff command is available, but unsupported scopes fail before
+	// suite factories or either provider runs.
+	outcome = app.Execute(context.Background(), []string{"config", "diff", "--scope", "plain"}, commandkit.Streams{Out: io.Discard, Err: io.Discard})
+	if outcome.Kind != commandkit.OutcomeUsage {
+		t.Fatalf("unsupported scope outcome = %#v", outcome)
+	}
+	if inputCalls != 0 || stateCalls != 0 || acquireCalls != 0 || compareCalls != 0 {
+		t.Fatalf("discovery or preflight invoked suite work: input=%d state=%d acquire=%d compare=%d", inputCalls, stateCalls, acquireCalls, compareCalls)
+	}
+	if captured.byName["app"].scope.Compare == nil {
+		t.Fatal("captured comparison callback was cleared by a source edit")
+	}
+	if _, err := captured.byName["app"].scope.Compare(context.Background(), ComparisonRequest[input]{}, struct{}{}); err != nil || compareCalls != 1 {
+		t.Fatalf("captured comparison callback result=%v calls=%d", err, compareCalls)
+	}
+
+	withoutComparison := newConfigTestApp(t, Command(Suite[input, struct{}]{Scopes: []Scope[input, struct{}]{{
+		Name: "plain", Operations: []Operation{Validate},
+		Acquire: func(context.Context, Request, input, struct{}) (commandkit.Acquired[View], error) {
+			return commandkit.Acquired[View]{}, nil
+		},
+	}}}))
+	outcome = withoutComparison.Execute(context.Background(), []string{"config", "diff"}, commandkit.Streams{Out: io.Discard, Err: io.Discard})
+	if outcome.Kind != commandkit.OutcomeUsage {
+		t.Fatalf("family without comparison exposed diff: %#v", outcome)
+	}
+}
+
+func TestComparisonCallbackWiringAndRetainedAdapterIssues(t *testing.T) {
+	newApp := func(scope Scope[struct{}, struct{}]) error {
+		_, err := commandkit.New(commandkit.Identity{Name: "example"}, Command(Suite[struct{}, struct{}]{Scopes: []Scope[struct{}, struct{}]{scope}}))
+		return err
+	}
+
+	if err := newApp(Scope[struct{}, struct{}]{Name: "app", Operations: []Operation{Diff}}); err == nil {
+		t.Fatalf("missing comparison callback error = %v", err)
+	}
+	if err := newApp(Scope[struct{}, struct{}]{Name: "app", Operations: []Operation{Validate}}); err == nil {
+		t.Fatalf("missing ordinary acquisition callback error = %v", err)
+	}
+
+	scope := FromConfigKit[struct{}, struct{}, struct{}]("app", "Application configuration", nil)
+	scope.Operations = []Operation{Diff}
+	scope.Compare = func(context.Context, ComparisonRequest[struct{}], struct{}) (commandkit.Acquired[[]configkit.Difference], error) {
+		return commandkit.Acquired[[]configkit.Difference]{}, nil
+	}
+	retained := scope.issues[0]
+	if err := newApp(scope); err == nil || !errors.Is(err, retained) {
+		t.Fatalf("narrowed adapter lost its retained loader issue: %v", err)
+	}
+}

integrations/ckconfig/contracts.go

@@ -29,6 +29,8 @@ const (
 	Sources Operation = "sources"
 	// Explain reports the history and provenance of a selected path.
 	Explain Operation = "explain"
+	// Diff compares two selections of the same configuration scope.
+	Diff Operation = "diff"
 )
 
 // Request describes one configuration operation and its selected inputs.
@@ -39,6 +41,24 @@ type Request struct {
 	Selection Selection
 }
 
+// ComparisonSide groups one configuration selection with its independently
+// prepared typed input.
+type ComparisonSide[I any] struct {
+	// Selection carries the explicit config and overlay inputs for this side.
+	Selection Selection
+	// Input is initialized and checked for this side before comparison.
+	Input I
+}
+
+// ComparisonRequest carries the earlier and later selections supplied to a
+// scope's comparison callback.
+type ComparisonRequest[I any] struct {
+	// Left is the earlier configuration selection.
+	Left ComparisonSide[I]
+	// Right is the later configuration selection.
+	Right ComparisonSide[I]
+}
+
 // EffectiveWriter writes a configuration's native effective representation.
 type EffectiveWriter interface {
 	WriteEffective(io.Writer) error
@@ -63,9 +83,11 @@ type View struct {
 	Explanation ExplanationReader
 }
 
-// Scope declares the operations and acquisition callback for one named
-// configuration. Construction issues are retained privately until family
-// assembly attaches them to its command declaration.
+// Scope declares the operations and providers for one named configuration.
+// Ordinary inspection operations use Acquire; Diff uses Compare and does not
+// require Acquire unless an ordinary operation is also declared. Construction
+// issues are retained privately until family assembly attaches them to its
+// command declaration.
 type Scope[I, S any] struct {
 	// Name identifies the scope in CLI selection and reports.
 	Name string
@@ -73,8 +95,12 @@ type Scope[I, S any] struct {
 	Summary string
 	// Operations selects the standard handlers exposed for this scope.
 	Operations []Operation
-	// Acquire loads the selected configuration for one inspection invocation.
+	// Acquire loads the selected configuration for one ordinary inspection
+	// invocation.
 	Acquire func(context.Context, Request, I, S) (commandkit.Acquired[View], error)
+	// Compare compares two selections for this scope using one shared state.
+	// It is required when Diff is declared in Operations.
+	Compare func(context.Context, ComparisonRequest[I], S) (commandkit.Acquired[[]configkit.Difference], error)
 
 	issues []error
 }

integrations/ckconfig/family.go

@@ -112,9 +112,20 @@ func captureSuite[I, S any](suite Suite[I, S]) *preparedSuite[I, S] {
 		for _, issue := range scope.issues {
 			prepared.issues = append(prepared.issues, fmt.Errorf("configuration scope %q: %w", scope.Name, issue))
 		}
-		if len(scope.Operations) > 0 && scope.Acquire == nil {
+		requiresAcquire, requiresCompare := false, false
+		for _, operation := range scope.Operations {
+			if operation == Diff {
+				requiresCompare = true
+			} else {
+				requiresAcquire = true
+			}
+		}
+		if requiresAcquire && scope.Acquire == nil {
 			prepared.issues = append(prepared.issues, fmt.Errorf("configuration scope %q declares operations without an acquisition callback", scope.Name))
 		}
+		if requiresCompare && scope.Compare == nil {
+			prepared.issues = append(prepared.issues, fmt.Errorf("configuration scope %q declares diff without a comparison callback", scope.Name))
+		}
 		prepared.scopes = append(prepared.scopes, &preparedScope[I, S]{scope: scope})
 	}
 	sort.SliceStable(prepared.scopes, func(i, j int) bool {
@@ -156,7 +167,7 @@ func captureSuite[I, S any](suite Suite[I, S]) *preparedSuite[I, S] {
 		}
 	}
 
-	for _, operation := range []Operation{Validate, Show, Sources, Explain} {
+	for _, operation := range []Operation{Validate, Show, Sources, Explain, Diff} {
 		for _, scope := range prepared.scopes {
 			if containsOperation(scope.scope.Operations, operation) {
 				prepared.operations = append(prepared.operations, operation)
@@ -185,14 +196,14 @@ func normalizeOperations(operations []Operation, issues *[]error, scope string)
 	seen := make(map[Operation]struct{}, len(operations))
 	for _, operation := range operations {
 		switch operation {
-		case Validate, Show, Sources, Explain:
+		case Validate, Show, Sources, Explain, Diff:
 		default:
 			*issues = append(*issues, fmt.Errorf("configuration scope %q declares unknown operation %q", scope, operation))
 		}
 		seen[operation] = struct{}{}
 	}
 	ordered := make([]Operation, 0, len(seen))
-	for _, operation := range []Operation{Validate, Show, Sources, Explain} {
+	for _, operation := range []Operation{Validate, Show, Sources, Explain, Diff} {
 		if _, exists := seen[operation]; exists {
 			ordered = append(ordered, operation)
 		}