thema: Tidy up base package and docs

assignable.go

@@ -15,8 +15,8 @@ import (
 // If the provided T is a pointer, it will be dereferenced before verification.
 // Double pointers (or any n-pointer > 1) are not allowed.
 //
-// The provided T must necessarily be of struct type, as it is a requirement
-// that all Thema schemas are of base type struct.
+// The provided T must struct-kinded, as it is a requirement that all Thema
+// schemas are of base type struct.
 //
 //	type MyType struct {
 //		MyField string `json:"myfield"`
@@ -32,9 +32,11 @@ func AssignableTo(sch Schema, T any) error {
 	return assignable(sch.Underlying().LookupPath(pathSchDef), T)
 }
 
-// ErrPointerDepth indicates that a Go type having pointer indirection depth > 1
-// (e.g. **struct{ V: string }) was provided to a Thema func that checks
-// assignability, such as [BindType].
+// ErrPointerDepth indicates that a Go type having pointer indirection depth greater than 1, such as
+//
+//	**struct{ V: string })
+//
+// was provided to a Thema func that checks assignability, such as [BindType].
 var ErrPointerDepth = errors.New("assignability does not support more than one level of pointer indirection")
 
 const scalarKinds = cue.NullKind | cue.BoolKind |

encoding/cue/encode.go

@@ -124,7 +124,7 @@ func appendlin(lin thema.Lineage, sch cue.Value) (ast.Node, error) {
 	linf := astutil.Format(lin.Underlying()).(*ast.File)
 	schnode := astutil.ToExpr(astutil.Format(sch))
 
-	lv := thema.LatestVersion(lin)
+	lv := lin.Latest().Version()
 	lsch := thema.SchemaP(lin, lv)
 	if err := compat.ThemaCompatible(lsch.Underlying(), sch); err == nil {
 		// Is compatible, bump minor version

errors/errors.go

@@ -1,6 +1,8 @@
 package errors
 
-import "github.com/cockroachdb/errors"
+import (
+	"github.com/cockroachdb/errors"
+)
 
 // ValidationCode represents different classes of validation errors that may
 // occur vs. concrete data inputs.

impl.go

@@ -7,27 +7,6 @@ import (
 	"cuelang.org/go/cue/errors"
 )
 
-// ErrValueNotExist indicates that an operation failed because a provided
-// cue.Value does not exist.
-type ErrValueNotExist struct {
-	path string
-}
-
-func (e *ErrValueNotExist) Error() string {
-	return fmt.Sprintf("value from path %q does not exist, absent values cannot be lineages", e.path)
-}
-
-// ErrNoSchemaWithVersion indicates that an operation was requested against a
-// schema version that does not exist within a particular lineage.
-type ErrNoSchemaWithVersion struct {
-	lin Lineage
-	v   SyntacticVersion
-}
-
-func (e *ErrNoSchemaWithVersion) Error() string {
-	return fmt.Sprintf("lineage %q does not contain a schema with version %v", e.lin.Name(), e.v)
-}
-
 type compatInvariantError struct {
 	rawlin    cue.Value
 	violation [2]SyntacticVersion

instance.go

@@ -25,15 +25,26 @@ func BindInstanceType[T Assignee](inst *Instance, tsch TypedSchema[T]) (*TypedIn
 	}, nil
 }
 
-// An Instance represents some data that has been validated against a
-// lineage's schema. It includes a reference to the schema.
+// Instance represents data that is a valid instance of a Thema [Schema].
+//
+// It is not possible to create a valid Instance directly. They can only be
+// obtained by successful call to [Schema.Validate].
 type Instance struct {
 	// The CUE representation of the input data
 	raw cue.Value
 	// A name for the input data, primarily for use in error messages
 	name string
 	// The schema the data validated against/of which the input data is a valid instance
 	sch Schema
+
+	// simple flag the prevents external creation
+	valid bool
+}
+
+func (i *Instance) check() {
+	if !i.valid {
+		panic("Instance is not valid; Instances must be created by a call to thema.Schema.Validate")
+	}
 }
 
 // Hydrate returns a copy of the Instance with all default values specified by
@@ -42,6 +53,8 @@ type Instance struct {
 // NOTE hydration implementation is a WIP. If errors are encountered, the
 // original input is returned unchanged.
 func (i *Instance) Hydrate() *Instance {
+	i.check()
+
 	i.sch.Lineage().Runtime()
 	ni, err := doHydrate(i.sch.Underlying(), i.raw)
 	// FIXME For now, just no-op it if we error
@@ -50,9 +63,10 @@ func (i *Instance) Hydrate() *Instance {
 	}
 
 	return &Instance{
-		raw:  ni,
-		name: i.name,
-		sch:  i.sch,
+		valid: true,
+		raw:   ni,
+		name:  i.name,
+		sch:   i.sch,
 	}
 }
 
@@ -62,66 +76,90 @@ func (i *Instance) Hydrate() *Instance {
 // NOTE dehydration implementation is a WIP. If errors are encountered, the
 // original input is returned unchanged.
 func (i *Instance) Dehydrate() *Instance {
+	i.check()
+
 	ni, _, err := doDehydrate(i.sch.Underlying(), i.raw)
 	// FIXME For now, just no-op it if we error
 	if err != nil {
 		return i
 	}
 
 	return &Instance{
-		raw:  ni,
-		name: i.name,
-		sch:  i.sch,
+		valid: true,
+		raw:   ni,
+		name:  i.name,
+		sch:   i.sch,
 	}
 }
 
 // AsSuccessor translates the instance into the form specified by the successor
 // schema.
-//
-// TODO figure out how to represent unary vs. composite lineages here
 func (i *Instance) AsSuccessor() (*Instance, TranslationLacunas) {
+	i.check()
 	return i.Translate(i.sch.Successor().Version())
 }
 
 // AsPredecessor translates the instance into the form specified by the predecessor
 // schema.
-//
-// TODO figure out how to represent unary vs. composite lineages here
 func (i *Instance) AsPredecessor() (*Instance, TranslationLacunas) {
-	panic("TODO translation from newer to older schema is not yet implemented")
+	i.check()
+	return i.Translate(i.sch.Predecessor().Version())
 }
 
-// Underlying returns the cue.Value representing the instance's underlying data.
+// Underlying returns the cue.Value representing the data contained in the Instance.
 func (i *Instance) Underlying() cue.Value {
+	i.check()
 	return i.raw
 }
 
-// Schema returns the schema which subsumes/validated this instance.
+// Schema returns the [Schema] corresponding to this instance.
 func (i *Instance) Schema() Schema {
+	i.check()
 	return i.sch
 }
 
 func (i *Instance) rt() *Runtime {
 	return getLinLib(i.Schema().Lineage())
 }
 
+// TypedInstance represents data that is a valid instance of a Thema
+// [TypedSchema].
+//
+// A TypedInstance is to a [TypedSchema] as an [Instance] is to a [Schema].
+//
+// It is not possible to create a valid TypedInstance directly. They can only be
+// obtained by successful call to [TypedSchema.Validate].
 type TypedInstance[T Assignee] struct {
 	*Instance
 	tsch TypedSchema[T]
 }
 
+// TypedSchema returns the [TypedSchema] corresponding to this instance.
+//
+// This method is identical to [Instance.Schema], except that it returns the already-typed variant.
 func (inst *TypedInstance[T]) TypedSchema() TypedSchema[T] {
+	inst.check()
 	return inst.tsch
 }
 
+// Value returns a Go struct of this TypedInstance's generic [Assignee] type,
+// populated with the data contained in this instance, including default values, etc.
+//
+// This method is similar to [json.Unmarshal] - it decodes serialized data into a standard Go type
+// for working with in all the usual ways.
 func (inst *TypedInstance[T]) Value() (T, error) {
+	inst.check()
+
 	t := inst.tsch.NewT()
 	// TODO figure out correct pointer handling here
 	err := inst.Instance.raw.Decode(&t)
 	return t, err
 }
 
+// ValueP is the same as Value, but panics if an error is encountered.
 func (inst *TypedInstance[T]) ValueP() T {
+	inst.check()
+
 	t, err := inst.Value()
 	if err != nil {
 		panic(fmt.Errorf("error decoding value: %w", err))
@@ -150,6 +188,8 @@ func (inst *TypedInstance[T]) ValueP() T {
 // achieved in the program depending on Thema, so we avoid introducing
 // complexity into Thema that is not essential for all use cases.
 func (i *Instance) Translate(to SyntacticVersion) (*Instance, TranslationLacunas) {
+	i.check()
+
 	// TODO define this in terms of AsSuccessor and AsPredecessor, rather than those in terms of this.
 	newsch, err := i.Schema().Lineage().Schema(to)
 	if err != nil {
@@ -179,9 +219,10 @@ func (i *Instance) Translate(to SyntacticVersion) (*Instance, TranslationLacunas
 	raw, _ := out.LookupPath(cue.MakePath(cue.Str("result"), cue.Str("result"))).Default()
 
 	return &Instance{
-		raw:  raw,
-		name: i.name,
-		sch:  newsch,
+		valid: true,
+		raw:   raw,
+		name:  i.name,
+		sch:   newsch,
 	}, lac
 }
 

lineage.go

@@ -6,6 +6,8 @@ import (
 
 	"cuelang.org/go/cue"
 	cerrors "cuelang.org/go/cue/errors"
+	"github.com/cockroachdb/errors"
+	terrors "github.com/grafana/thema/errors"
 	"github.com/grafana/thema/internal/cuetil"
 )
 
@@ -40,7 +42,7 @@ type baseLineage struct {
 
 // BindLineage takes a raw [cue.Value], checks that it correctly follows Thema's
 // invariants, such as translatability and backwards compatibility version
-// numbering. If checks succeed, a [Lineage] is returned.
+// numbering. If these checks succeed, a [Lineage] is returned.
 //
 // This function is the only way to create non-nil Lineage objects. As a result,
 // all non-nil instances of Lineage in any Go program are guaranteed to follow
@@ -247,10 +249,7 @@ func (lin *baseLineage) Schema(v SyntacticVersion) (Schema, error) {
 	isValidLineage(lin)
 
 	if !synvExists(lin.allv, v) {
-		return nil, &ErrNoSchemaWithVersion{
-			lin: lin,
-			v:   v,
-		}
+		return nil, errors.Mark(errors.Newf("no schema with version %s in lineage %s", v, lin.name), terrors.ErrVersionNotExist)
 	}
 
 	return lin.schema(v), nil

schema.go

@@ -48,9 +48,10 @@ func (sch *schemaDef) Examples() map[string]*Instance {
 	for it.Next() {
 		label := it.Selector().String()
 		examples[label] = &Instance{
-			raw:  it.Value(),
-			name: label,
-			sch:  sch,
+			valid: true,
+			raw:   it.Value(),
+			name:  label,
+			sch:   sch,
 		}
 	}
 
@@ -86,9 +87,10 @@ func (sch *schemaDef) Validate(data cue.Value) (*Instance, error) {
 	}
 
 	return &Instance{
-		raw:  data,
-		sch:  sch,
-		name: "", // FIXME how are we getting this out?
+		valid: true,
+		raw:   data,
+		sch:   sch,
+		name:  "", // FIXME how are we getting this out?
 	}, nil
 }
 

surface.go

@@ -99,7 +99,7 @@ func SchemaP(lin Lineage, v SyntacticVersion) Schema {
 // LatestVersion returns the version number of the newest (largest) schema
 // version in the provided lineage.
 //
-// DEPRECATED: call Lineage.Latest().Version().
+// Deprecated: call Lineage.Latest().Version().
 func LatestVersion(lin Lineage) SyntacticVersion {
 	return lin.Latest().Version()
 }
@@ -109,7 +109,7 @@ func LatestVersion(lin Lineage) SyntacticVersion {
 //
 // An error indicates the number of the provided sequence does not exist.
 //
-// DEPRECATED: call Schema.LatestInMajor().Version() after loading a schema in the desired major version.
+// Deprecated: call Schema.LatestInMajor().Version() after loading a schema in the desired major version.
 func LatestVersionInSequence(lin Lineage, seqv uint) (SyntacticVersion, error) {
 	sch, err := lin.Schema(SV(seqv, 0))
 	if err != nil {
@@ -135,6 +135,8 @@ func LatestVersionInSequence(lin Lineage, seqv uint) (SyntacticVersion, error) {
 // the builder func to reduce stutter:
 //
 //	func Lineage ...
+//
+// Deprecated: having an explicit type for this adds little value.
 type LineageFactory func(*Runtime, ...BindOption) (Lineage, error)
 
 // A ConvergentLineageFactory is the same as a LineageFactory, but for a
@@ -143,6 +145,8 @@ type LineageFactory func(*Runtime, ...BindOption) (Lineage, error)
 // There is no reason to provide both a ConvergentLineageFactory and a
 // LineageFactory, as the latter is always reachable from the former. As such,
 // idiomatic naming conventions are unchanged.
+//
+// Deprecated: having an explicit type for this adds little value.
 type ConvergentLineageFactory[T Assignee] func(*Runtime, ...BindOption) (ConvergentLineage[T], error)
 
 // A BindOption defines options that may be specified only at initial