gozcl: DecodeExpression implementation

This is really just a wrapper around converting the value to a Go native
value with gocty, but is convenient to provide since it can attempt
type conversion and produce diagnostics automatically.

gozcl/decode.go

@@ -1,6 +1,10 @@
 package gozcl
 
 import (
+	"fmt"
+
+	"github.com/apparentlymart/go-cty/cty/convert"
+	"github.com/apparentlymart/go-cty/cty/gocty"
 	"github.com/apparentlymart/go-zcl/zcl"
 )
 
@@ -40,6 +44,35 @@ func DecodeBody(body zcl.Body, ctx *zcl.EvalContext, val interface{}) zcl.Diagno
 // may still be accessed by a careful caller for static analysis and editor
 // integration use-cases.
 func DecodeExpression(expr zcl.Expression, ctx *zcl.EvalContext, val interface{}) zcl.Diagnostics {
-	// TODO: Implement
-	return nil
+	srcVal, diags := expr.Value(ctx)
+
+	convTy, err := gocty.ImpliedType(val)
+	if err != nil {
+		panic(fmt.Sprintf("unsuitable DecodeExpression target: %s", err))
+	}
+
+	srcVal, err = convert.Convert(srcVal, convTy)
+	if err != nil {
+		diags = append(diags, &zcl.Diagnostic{
+			Severity: zcl.DiagError,
+			Summary:  "Unsuitable value type",
+			Detail:   fmt.Sprintf("Incorrect value type: %s", err.Error()),
+			Subject:  expr.StartRange().Ptr(),
+			Context:  expr.Range().Ptr(),
+		})
+		return diags
+	}
+
+	err = gocty.FromCtyValue(srcVal, val)
+	if err != nil {
+		diags = append(diags, &zcl.Diagnostic{
+			Severity: zcl.DiagError,
+			Summary:  "Unsuitable value type",
+			Detail:   fmt.Sprintf("Incorrect value type: %s", err.Error()),
+			Subject:  expr.StartRange().Ptr(),
+			Context:  expr.Range().Ptr(),
+		})
+	}
+
+	return diags
 }

gozcl/decode_test.go

@@ -0,0 +1,97 @@
+package gozcl
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/apparentlymart/go-cty/cty"
+	"github.com/apparentlymart/go-zcl/zcl"
+)
+
+func TestDecodeExpression(t *testing.T) {
+	tests := []struct {
+		Value     cty.Value
+		Target    interface{}
+		Want      interface{}
+		DiagCount int
+	}{
+		{
+			cty.StringVal("hello"),
+			"",
+			"hello",
+			0,
+		},
+		{
+			cty.StringVal("hello"),
+			cty.NilVal,
+			cty.StringVal("hello"),
+			0,
+		},
+		{
+			cty.NumberIntVal(2),
+			"",
+			"2",
+			0,
+		},
+		{
+			cty.StringVal("true"),
+			false,
+			true,
+			0,
+		},
+		{
+			cty.NullVal(cty.String),
+			"",
+			"",
+			1, // null value is not allowed
+		},
+		{
+			cty.UnknownVal(cty.String),
+			"",
+			"",
+			1, // value must be known
+		},
+		{
+			cty.ListVal([]cty.Value{cty.True}),
+			false,
+			false,
+			1, // bool required
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("%02d", i), func(t *testing.T) {
+			expr := &fixedExpression{test.Value}
+
+			targetVal := reflect.New(reflect.TypeOf(test.Target))
+
+			diags := DecodeExpression(expr, nil, targetVal.Interface())
+			if len(diags) != test.DiagCount {
+				t.Errorf("wrong number of diagnostics %d; want %d", len(diags), test.DiagCount)
+				for _, diag := range diags {
+					t.Logf(" - %s", diag.Error())
+				}
+			}
+			got := targetVal.Elem().Interface()
+			if !reflect.DeepEqual(got, test.Want) {
+				t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, test.Want)
+			}
+		})
+	}
+}
+
+type fixedExpression struct {
+	val cty.Value
+}
+
+func (e *fixedExpression) Value(ctx *zcl.EvalContext) (cty.Value, zcl.Diagnostics) {
+	return e.val, nil
+}
+
+func (e *fixedExpression) Range() (r zcl.Range) {
+	return
+}
+func (e *fixedExpression) StartRange() (r zcl.Range) {
+	return
+}