Added support for custom unmarshalling.

go.mod

@@ -15,7 +15,7 @@ require (
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/sergi/go-diff v1.0.0
 	github.com/spf13/pflag v1.0.2
-	github.com/stretchr/testify v1.2.2 // indirect
+	github.com/stretchr/testify v1.5.1
 	github.com/zclconf/go-cty v1.2.0
 	golang.org/x/crypto v0.0.0-20190426145343-a29dc8fdc734
 	golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82 // indirect

go.sum

@@ -6,6 +6,7 @@ github.com/apparentlymart/go-textseg v1.0.0 h1:rRmlIsPEEhUTIKQb7T++Nz/A5Q6C9IuX2
 github.com/apparentlymart/go-textseg v1.0.0/go.mod h1:z96Txxhf3xSFMPmb5X/1W05FF/Nj9VFpLOpjS5yuumk=
 github.com/apparentlymart/go-textseg/v12 v12.0.0 h1:bNEQyAGak9tojivJNkoqWErVCQbjdL7GzRt3F8NvfJ0=
 github.com/apparentlymart/go-textseg/v12 v12.0.0/go.mod h1:S/4uRK2UtaQttw1GenVJEynmyUenKwP++x/+DdGV/Ec=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/go-test/deep v1.0.3 h1:ZrJSEWsXzPOxaZnFteGEfooLba+ju3FYIbOrS+rQd68=
@@ -28,8 +29,12 @@ github.com/sergi/go-diff v1.0.0 h1:Kpca3qRNrduNnOQeazBd0ysaKrUJiIuISHxogkT9RPQ=
 github.com/sergi/go-diff v1.0.0/go.mod h1:0CfEIISq7TuYL3j771MWULgwwjU+GofnZX9QAmXWZgo=
 github.com/spf13/pflag v1.0.2 h1:Fy0orTDgHdbnzHcsOgfCN4LtHf0ec3wwtiwJqwvf3Gc=
 github.com/spf13/pflag v1.0.2/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
+github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.2.2 h1:bSDNvY7ZPG5RlJ8otE/7V6gMiyenm9RtJ7IUVIAoJ1w=
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
+github.com/stretchr/testify v1.5.1 h1:nOGnQDM7FYENwehXlg/kFVnos3rEvtKTjRvOWSzb6H4=
+github.com/stretchr/testify v1.5.1/go.mod h1:5W2xD1RspED5o8YsWQXVCued0rvSQ+mT+I5cxcmMvtA=
 github.com/vmihailenco/msgpack v3.3.3+incompatible/go.mod h1:fy3FlTQTDXWkZ7Bh6AcGMlsjHatGryHQYUTf1ShIgkk=
 github.com/zclconf/go-cty v1.2.0 h1:sPHsy7ADcIZQP3vILvTjrh74ZA175TFP5vqiNK1UmlI=
 github.com/zclconf/go-cty v1.2.0/go.mod h1:hOPWgoHbaTUnI5k4D2ld+GRpFJSCe6bCM7m1q/N4PQ8=
@@ -49,5 +54,8 @@ golang.org/x/text v0.3.2 h1:tW2bmiBqwgJj/UpqtC8EpXEZVYOwU0yG4iWbprSVAcs=
 golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 google.golang.org/appengine v1.1.0/go.mod h1:EbEs0AVv82hx2wNQdGPgUI5lhzA/G0D9YwlJXL52JkM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
+gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=

gohcl/decode.go

@@ -36,6 +36,10 @@ func DecodeBody(body hcl.Body, ctx *hcl.EvalContext, val interface{}) hcl.Diagno
 	return decodeBodyToValue(body, ctx, rv.Elem())
 }
 
+type Unmarshaler interface {
+	UnmarshalHCL(block *hcl.Block, ctx *hcl.EvalContext) hcl.Diagnostics
+}
+
 func decodeBodyToValue(body hcl.Body, ctx *hcl.EvalContext, val reflect.Value) hcl.Diagnostics {
 	et := val.Type()
 	switch et.Kind() {
@@ -246,6 +250,18 @@ func decodeBlockToValue(block *hcl.Block, ctx *hcl.EvalContext, v reflect.Value)
 
 	ty := v.Type()
 
+	if v.CanInterface() {
+		if unm, ok := v.Interface().(Unmarshaler); ok {
+			return unm.UnmarshalHCL(block, ctx)
+		}
+	}
+
+	if v.CanAddr() && v.CanInterface() {
+		if unm, ok := v.Addr().Interface().(Unmarshaler); ok {
+			return unm.UnmarshalHCL(block, ctx)
+		}
+	}
+
 	switch {
 	case blockType.AssignableTo(ty):
 		v.Elem().Set(reflect.ValueOf(block))

gohcl/decode_test.go

@@ -3,15 +3,73 @@ package gohcl
 import (
 	"encoding/json"
 	"fmt"
-	"reflect"
-	"testing"
-
 	"github.com/davecgh/go-spew/spew"
 	"github.com/kage-cloud/hcl/v2"
 	hclJSON "github.com/kage-cloud/hcl/v2/json"
+	"github.com/stretchr/testify/assert"
 	"github.com/zclconf/go-cty/cty"
+	"reflect"
+	"testing"
 )
 
+type withTwoAttributes struct {
+	A string `hcl:"a,optional" json:"a"`
+	B string `hcl:"b,optional" json:"b"`
+}
+
+func (w *withTwoAttributes) UnmarshalHCL(block *hcl.Block, ctx *hcl.EvalContext) hcl.Diagnostics {
+	if diags := DecodeBody(block.Body, ctx, w); diags != nil && diags.HasErrors() {
+		return diags
+	}
+
+	t := w.A
+	w.A = w.B
+	w.B = t
+
+	return nil
+}
+
+type testStruct struct {
+	Attributes []withTwoAttributes `hcl:"attrs,block" json:"attrs"`
+}
+
+func TestUnmarshal(t *testing.T) {
+	given := &testStruct{
+		Attributes: []withTwoAttributes{
+			{
+				A: "hello",
+				B: "world",
+			},
+		},
+	}
+
+	expected := given
+
+	b, err := json.Marshal(given)
+	if err != nil {
+		fmt.Println(err.Error())
+		t.FailNow()
+	}
+
+	expected.Attributes[0].A = "world"
+	expected.Attributes[0].B = "hello"
+
+	f, diags := hclJSON.Parse(b, "test.json")
+	if diags != nil && diags.HasErrors() {
+		fmt.Println(diags.Error())
+		t.FailNow()
+	}
+
+	actual := new(testStruct)
+	diags = DecodeBody(f.Body, nil, actual)
+	if diags != nil && diags.HasErrors() {
+		fmt.Println(diags.Error())
+		t.FailNow()
+	}
+
+	assert.Equal(t, expected, actual)
+}
+
 func TestDecodeBody(t *testing.T) {
 	deepEquals := func(other interface{}) func(v interface{}) bool {
 		return func(v interface{}) bool {