encoding/toml: first decoder implementation

Multiple pieces are left to be done, such as tables with headers,
comments, node positions, and error positions.
However, this is already a significant enough amount of code and tests
which can be reviewed and merged on its own.

The Go API will remain experimental for the foreseeable future.
We will hook up this encoding to cmd/cue and the standard library
once it is more complete, particularly once we have an encoder.

Updates #68.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I4daca2d0e34e60ef891233872ae978a5df0ebedd

encoding/toml/decode.go

@@ -0,0 +1,177 @@
+// Copyright 2024 The CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package toml converts TOML to and from CUE.
+//
+// WARNING: THIS PACKAGE IS EXPERIMENTAL.
+// ITS API MAY CHANGE AT ANY TIME.
+package toml
+
+import (
+	"fmt"
+	"io"
+
+	toml "github.com/pelletier/go-toml/v2/unstable"
+
+	"cuelang.org/go/cue/ast"
+	"cuelang.org/go/cue/token"
+)
+
+// TODO(mvdan): filename, schema, and decode options
+
+// NewDecoder creates a decoder from a stream of TOML input.
+func NewDecoder(r io.Reader) *Decoder {
+	// Note that we don't consume the reader here,
+	// as there's no need, and we can't return an error either.
+	return &Decoder{r: r}
+}
+
+// Decoder implements the decoding state.
+//
+// Note that TOML files and streams never decode multiple CUE nodes;
+// subsequent calls to [Decoder.Decode] may return [io.EOF].
+type Decoder struct {
+	r io.Reader
+
+	decoded bool // whether [Decoder.Decoded] has been called already
+	parser  toml.Parser
+
+	currentFields []*ast.Field
+}
+
+// TODO(mvdan): support decoding comments
+// TODO(mvdan): support ast.Node positions
+// TODO(mvdan): support error positions
+
+// Decode parses the input stream as TOML and converts it to a CUE [*ast.File].
+// Because TOML files only contain a single top-level expression,
+// subsequent calls to this method may return [io.EOF].
+func (d *Decoder) Decode() (ast.Node, error) {
+	if d.decoded {
+		return nil, io.EOF
+	}
+	d.decoded = true
+	// TODO(mvdan): unfortunately go-toml does not support streaming as of v2.2.2.
+	data, err := io.ReadAll(d.r)
+	if err != nil {
+		return nil, err
+	}
+	d.parser.Reset(data)
+	file := &ast.File{}
+	for d.parser.NextExpression() {
+		if err := d.nextRootNode(d.parser.Expression()); err != nil {
+			return nil, err
+		}
+	}
+	for _, field := range d.currentFields {
+		file.Decls = append(file.Decls, field)
+	}
+	d.currentFields = d.currentFields[:0]
+	if len(file.Decls) == 0 {
+		// Empty inputs are decoded as null, much like JSON or YAML.
+		file.Decls = append(file.Decls, ast.NewNull())
+	}
+	return file, nil
+}
+
+// nextRootNode is called for every top-level expression from the TOML parser.
+//
+// This method does not return a syntax tree node directly,
+// because some kinds of top-level expressions like comments and table headers
+// require recording some state in the decoder to produce a node at a later time.
+func (d *Decoder) nextRootNode(tnode *toml.Node) error {
+	switch tnode.Kind {
+	// Key-Values in TOML are in the form of:
+	//
+	//   foo.title   = "Foo"
+	//   foo.bar.baz = "value"
+	//
+	// We can decode them as "inline" structs in CUE, which keeps the original shape:
+	//
+	//   foo: title:    "Foo"
+	//   foo: bar: baz: "value"
+	//
+	// An alternative would be to join struct literals, which avoids some repetition,
+	// but also introduces extra lines and may break some comment positions:
+	//
+	//   foo: {
+	//       title:    "Foo"
+	//       bar: baz: "value"
+	//   }
+	case toml.KeyValue:
+		keys := tnode.Key()
+		topField := &ast.Field{
+			Label: &ast.Ident{
+				NamePos: token.NoPos.WithRel(token.Newline),
+				Name:    string(keys.Node().Data),
+			},
+		}
+		ast.SetRelPos(topField.Label, token.Newline)
+		keys.Next() // TODO(mvdan): for some reason the first Next call doesn't count?
+		curField := topField
+		for keys.Next() {
+			nextField := &ast.Field{
+				Label: &ast.Ident{
+					NamePos: token.NoPos.WithRel(token.Blank),
+					Name:    string(keys.Node().Data),
+				},
+			}
+			ast.SetRelPos(nextField.Label, token.Blank)
+			curField.Value = &ast.StructLit{Elts: []ast.Decl{nextField}}
+			curField = nextField
+		}
+		value, err := d.decodeExpr(tnode.Value())
+		if err != nil {
+			return err
+		}
+		curField.Value = value
+		d.currentFields = append(d.currentFields, topField)
+	// TODO(mvdan): tables
+	// TODO(mvdan): array tables
+	default:
+		return fmt.Errorf("encoding/toml.Decoder.nextRootNode: unknown %s %#v\n", tnode.Kind, tnode)
+	}
+	return nil
+}
+
+// nextRootNode is called for every top-level expression from the TOML parser.
+func (d *Decoder) decodeExpr(tnode *toml.Node) (ast.Expr, error) {
+	data := string(tnode.Data)
+	switch tnode.Kind {
+	case toml.String:
+		return ast.NewString(data), nil
+	case toml.Integer:
+		return ast.NewLit(token.INT, data), nil
+	case toml.Float:
+		return ast.NewLit(token.FLOAT, data), nil
+	case toml.Bool:
+		return ast.NewBool(data == "true"), nil
+	case toml.Array:
+		list := &ast.ListLit{}
+		elems := tnode.Children()
+		for elems.Next() {
+			elem, err := d.decodeExpr(elems.Node())
+			if err != nil {
+				return nil, err
+			}
+			list.Elts = append(list.Elts, elem)
+		}
+		return list, nil
+	// TODO(mvdan): dates and times
+	// TODO(mvdan): inline tables
+	default:
+		return nil, fmt.Errorf("encoding/toml.Decoder.decodeExpr: unknown %s %#v\n", tnode.Kind, tnode)
+	}
+	return nil, nil
+}