style: rename repr to expr (#44)

## Description

The `Repr` type name was unclear — it was intended to represent a lambda calculus expression, not a "representation."
This PR renames `Repr` to `Expr` throughout the registry package for clarity.

- Rename `Repr` interface to `Expr` and `baseRepr` struct to `baseExpr`.
- Rename `repr.go` to `expr.go`.
- Rename `ID()` method to `Repr()` to indicate the representation type.
- Rename `NewRepr` constructor to `NewExpr`.
- Update all usages in codec, conversion, engine, process, and registry files.
- Add command aliases `conv` and `eng` for `convert` and `engine` subcommands.

## Benefits

- The naming better reflects the domain: an `Expr` is an expression, and `Repr()` returns its representation kind.
- Command aliases reduce typing for common subcommands.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #44
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

internal/registry/codec.go

@@ -8,7 +8,7 @@ import (
 )
 
 type Codec interface {
-	codec.Codec[Repr]
+	codec.Codec[Expr]
 
 	InType() string
 }
@@ -18,16 +18,16 @@ type convertedCodec[T any] struct {
 	inType string
 }
 
-func (c convertedCodec[T]) Decode(s string) (Repr, error) {
+func (c convertedCodec[T]) Decode(s string) (Expr, error) {
 	t, err := c.codec.Decode(s)
 	if err != nil {
 		return nil, err
 	}
 
-	return NewRepr(c.inType, t), nil
+	return NewExpr(c.inType, t), nil
 }
 
-func (c convertedCodec[T]) Encode(r Repr) (string, error) {
+func (c convertedCodec[T]) Encode(r Expr) (string, error) {
 	t, ok := r.Data().(T)
 	if !ok {
 		dataType := reflect.TypeOf(r.Data())

internal/registry/conversion.go

@@ -10,7 +10,7 @@ type Conversion interface {
 	InType() string
 	OutType() string
 
-	Run(Repr) (Repr, error)
+	Run(Expr) (Expr, error)
 }
 
 type convertedConversion[T, U any] struct {
@@ -18,8 +18,8 @@ type convertedConversion[T, U any] struct {
 	inType, outType string
 }
 
-func (c convertedConversion[T, U]) Run(r Repr) (Repr, error) {
-	t, ok := r.Data().(T)
+func (c convertedConversion[T, U]) Run(expr Expr) (Expr, error) {
+	t, ok := expr.Data().(T)
 	if !ok {
 		return nil, fmt.Errorf("could not parse '%v' as '%s'", t, c.inType)
 	}
@@ -29,7 +29,7 @@ func (c convertedConversion[T, U]) Run(r Repr) (Repr, error) {
 		return nil, err
 	}
 
-	return NewRepr(c.outType, u), nil
+	return NewExpr(c.outType, u), nil
 }
 
 func (c convertedConversion[T, U]) InType() string { return c.inType }

internal/registry/engine.go

@@ -7,7 +7,7 @@ import (
 )
 
 type Engine interface {
-	Load(Repr) (Process, error)
+	Load(Expr) (Process, error)
 	Name() string
 	InType() string
 }
@@ -22,10 +22,10 @@ func (e convertedEngine[T]) InType() string { return e.inType }
 
 func (e convertedEngine[T]) Name() string { return e.name }
 
-func (e convertedEngine[T]) Load(r Repr) (Process, error) {
-	t, ok := r.Data().(T)
+func (e convertedEngine[T]) Load(expr Expr) (Process, error) {
+	t, ok := expr.Data().(T)
 	if !ok {
-		return nil, fmt.Errorf("'ncorrent format '%s' for engine '%s'", r.ID(), e.inType)
+		return nil, fmt.Errorf("'ncorrent format '%s' for engine '%s'", expr.Repr(), e.inType)
 	}
 
 	process, err := e.engine(t)

internal/registry/expr.go

@@ -0,0 +1,24 @@
+package registry
+
+// A Expr is a lambda calculus expression. It can have any type of
+// Expresentation, so long as that class is known to the registry it is handled
+// by.
+type Expr interface {
+	// Repr returns the name of the underlying Expresentation. It is assumed if
+	// two expressions have the same Repr(), they have the same Expresentation.
+	Repr() string
+
+	// The base expression data.
+	Data() any
+}
+
+type baseExpr struct {
+	id   string
+	data any
+}
+
+func (r baseExpr) Repr() string { return r.id }
+
+func (r baseExpr) Data() any { return r.data }
+
+func NewExpr(id string, data any) Expr { return baseExpr{id, data} }

internal/registry/process.go

@@ -5,7 +5,7 @@ import (
 )
 
 type Process interface {
-	engine.Process[Repr]
+	engine.Process[Expr]
 
 	InType() string
 }
@@ -17,13 +17,13 @@ type convertedProcess[T any] struct {
 
 func (e convertedProcess[T]) InType() string { return e.inType }
 
-func (b convertedProcess[T]) Get() (Repr, error) {
+func (b convertedProcess[T]) Get() (Expr, error) {
 	s, err := b.process.Get()
 	if err != nil {
 		return nil, err
 	}
 
-	return NewRepr(b.inType, s), nil
+	return NewExpr(b.inType, s), nil
 }
 
 func (b convertedProcess[T]) Step(i int) bool {

internal/registry/registry.go

@@ -40,16 +40,18 @@ func (r *Registry) GetDefaultEngine(id string) (Engine, error) {
 		}
 	}
 
-	return nil, fmt.Errorf("no engine for '%s'", id)
+	return r.GetEngine("normalorder")
+
+	// return nil, fmt.Errorf("no engine for '%s'", id)
 }
 
-func (r *Registry) ConvertTo(repr Repr, outType string) (Repr, error) {
-	path, err := r.ConversionPath(repr.ID(), outType)
+func (r *Registry) ConvertTo(expr Expr, outType string) (Expr, error) {
+	path, err := r.ConversionPath(expr.Repr(), outType)
 	if err != nil {
 		return nil, err
 	}
 
-	result := repr
+	result := expr
 	for _, conversion := range path {
 		result, err = conversion.Run(result)
 		if err != nil {
@@ -60,16 +62,16 @@ func (r *Registry) ConvertTo(repr Repr, outType string) (Repr, error) {
 	return result, err
 }
 
-func (r *Registry) Marshal(repr Repr) (string, error) {
-	m, ok := r.codecs[repr.ID()]
+func (r *Registry) Marshal(expr Expr) (string, error) {
+	m, ok := r.codecs[expr.Repr()]
 	if !ok {
-		return "", fmt.Errorf("no marshaler for '%s'", repr.ID())
+		return "", fmt.Errorf("no marshaler for '%s'", expr.Repr())
 	}
 
-	return m.Encode(repr)
+	return m.Encode(expr)
 }
 
-func (r *Registry) Unmarshal(s string, outType string) (Repr, error) {
+func (r *Registry) Unmarshal(s string, outType string) (Expr, error) {
 	m, ok := r.codecs[outType]
 	if !ok {
 		return nil, fmt.Errorf("no marshaler for '%s'", outType)

internal/registry/repr.go

@@ -1,24 +0,0 @@
-package registry
-
-// A Repr is a lambda calculus expression. It can have any type of
-// representation, so long as that class is known to the registry it is handled
-// by.
-type Repr interface {
-	// ID returns the name of the underlying representation. It is assumed that
-	// if two expressions have the same Id(), they have the same representation.
-	ID() string
-
-	// The base expression data.
-	Data() any
-}
-
-type baseRepr struct {
-	id   string
-	data any
-}
-
-func (r baseRepr) ID() string { return r.id }
-
-func (r baseRepr) Data() any { return r.data }
-
-func NewRepr(id string, data any) Repr { return baseRepr{id, data} }