hclwrite: NewEmptyFile function

This completes the minimal functionality for creating a new file from
scratch, rather than modifying an existing one. This is illustrated by
a new test TestRoundupCreate that uses the API to create a new file in a
similar way to how a calling application might.

hclwrite/ast.go

@@ -12,6 +12,17 @@ type File struct {
 	body     *node
 }
 
+// NewEmptyFile constructs a new file with no content, ready to be mutated
+// by other calls that append to its body.
+func NewEmptyFile() *File {
+	f := &File{
+		inTree: newInTree(),
+	}
+	body := newBody()
+	f.body = f.children.Append(body)
+	return f
+}
+
 // Body returns the root body of the file, which contains the top-level
 // attributes and blocks.
 func (f *File) Body() *Body {

hclwrite/roundup_test.go

@@ -0,0 +1,66 @@
+package hclwrite_test
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/hashicorp/hcl2/hclwrite"
+	"github.com/zclconf/go-cty/cty"
+)
+
+// TestRoundupCreate is a test that exercises a number of different codepaths
+// to create a file from scratch, like a calling application might.
+func TestRoundupCreate(t *testing.T) {
+	f := hclwrite.NewEmptyFile()
+	rootBody := f.Body()
+	rootBody.SetAttributeValue("string", cty.StringVal("bar")) // this is overwritten later
+	rootBody.AppendNewline()
+	rootBody.SetAttributeValue("object", cty.ObjectVal(map[string]cty.Value{
+		"foo": cty.StringVal("foo"),
+		"bar": cty.NumberIntVal(5),
+		"baz": cty.True,
+	}))
+	rootBody.SetAttributeValue("string", cty.StringVal("foo"))
+	rootBody.SetAttributeValue("bool", cty.False)
+	rootBody.AppendNewline()
+	fooBlock := rootBody.AppendNewBlock("foo", nil)
+	fooBody := fooBlock.Body()
+	rootBody.AppendNewBlock("empty", nil)
+	rootBody.AppendNewline()
+	barBlock := rootBody.AppendNewBlock("bar", []string{"a", "b"})
+	barBody := barBlock.Body()
+
+	fooBody.SetAttributeValue("hello", cty.StringVal("world"))
+
+	bazBlock := barBody.AppendNewBlock("baz", nil)
+	bazBody := bazBlock.Body()
+	bazBody.SetAttributeValue("foo", cty.NumberIntVal(10))
+	bazBody.SetAttributeValue("beep", cty.StringVal("boop"))
+	bazBody.SetAttributeValue("baz", cty.ListValEmpty(cty.String))
+
+	got := string(bytes.TrimSpace(f.Bytes()))
+	want := strings.TrimSpace(`
+string = "foo"
+
+object = {bar = 5, baz = true, foo = "foo"}
+bool   = false
+
+foo {
+  hello = "world"
+}
+empty {
+}
+
+bar "a" "b" {
+  baz {
+    foo  = 10
+    beep = "boop"
+    baz  = []
+  }
+}
+`)
+	if got != want {
+		t.Errorf("wrong result\ngot:\n%s\n\nwant:\n%s", got, want)
+	}
+}