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)