hcldec: New DefaultSpec specification

This is a wrapper that allows a default value to be applied if a primary
spec results in a null value.

hcldec/public_test.go

@@ -5,8 +5,8 @@ import (
 	"reflect"
 	"testing"
 
-	"github.com/hashicorp/hcl2/hcl/hclsyntax"
 	"github.com/hashicorp/hcl2/hcl"
+	"github.com/hashicorp/hcl2/hcl/hclsyntax"
 	"github.com/zclconf/go-cty/cty"
 )
 
@@ -56,6 +56,36 @@ func TestDecode(t *testing.T) {
 			cty.NumberIntVal(1),
 			0,
 		},
+		{
+			"a = 1\n",
+			&DefaultSpec{
+				Primary: &AttrSpec{
+					Name: "a",
+					Type: cty.Number,
+				},
+				Default: &LiteralSpec{
+					Value: cty.NumberIntVal(10),
+				},
+			},
+			nil,
+			cty.NumberIntVal(1),
+			0,
+		},
+		{
+			"",
+			&DefaultSpec{
+				Primary: &AttrSpec{
+					Name: "a",
+					Type: cty.Number,
+				},
+				Default: &LiteralSpec{
+					Value: cty.NumberIntVal(10),
+				},
+			},
+			nil,
+			cty.NumberIntVal(10),
+			0,
+		},
 		{
 			"a = \"1\"\n",
 			&AttrSpec{

hcldec/spec.go

@@ -566,3 +566,33 @@ func (s *BlockLabelSpec) sourceRange(content *hcl.BodyContent, block *hcl.Block)
 
 	return block.LabelRanges[s.Index]
 }
+
+// DefaultSpec is a spec that wraps two specs, evaluating the primary first
+// and then evaluating the default if the primary returns a null value.
+type DefaultSpec struct {
+	Primary Spec
+	Default Spec
+}
+
+func (s *DefaultSpec) visitSameBodyChildren(cb visitFunc) {
+	cb(s.Primary)
+	cb(s.Default)
+}
+
+func (s *DefaultSpec) decode(content *hcl.BodyContent, block *hcl.Block, ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
+	val, diags := s.Primary.decode(content, block, ctx)
+	if val.IsNull() {
+		var moreDiags hcl.Diagnostics
+		val, moreDiags = s.Default.decode(content, block, ctx)
+		diags = append(diags, moreDiags...)
+	}
+	return val, diags
+}
+
+func (s *DefaultSpec) sourceRange(content *hcl.BodyContent, block *hcl.Block) hcl.Range {
+	// We can't tell from here which of the two specs will ultimately be used
+	// in our result, so we'll just assume the first. This is usually the right
+	// choice because the default is often a literal spec that doesn't have a
+	// reasonable source range to return anyway.
+	return s.Primary.sourceRange(content, block)
+}

hcldec/spec_test.go

@@ -11,3 +11,4 @@ var blockListSpecAsSpec Spec = (*BlockListSpec)(nil)
 var blockSetSpecAsSpec Spec = (*BlockSetSpec)(nil)
 var blockMapSpecAsSpec Spec = (*BlockMapSpec)(nil)
 var blockLabelSpecAsSpec Spec = (*BlockLabelSpec)(nil)
+var defaultSepcAsSpec Spec = (*DefaultSpec)(nil)