json: Add ParseExpression function

3 files changed, 164 insertions, 0 deletions

diff --git a/json/parser.go b/json/parser.go
index 4e40068..6b7420b 100644
--- a/json/parser.go
+++ b/json/parser.go
@@ -23,6 +23,21 @@ func parseFileContent(buf []byte, filename string, start hcl.Pos) (node, hcl.Dia
 	return node, diags
 }
 
+func parseExpression(buf []byte, filename string, start hcl.Pos) (node, hcl.Diagnostics) {
+	tokens := scan(buf, pos{Filename: filename, Pos: start})
+	p := newPeeker(tokens)
+	node, diags := parseValue(p)
+	if len(diags) == 0 && p.Peek().Type != tokenEOF {
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Extraneous data after value",
+			Detail:   "Extra characters appear after the JSON value.",
+			Subject:  p.Peek().Range.Ptr(),
+		})
+	}
+	return node, diags
+}
+
 func parseValue(p *peeker) (node, hcl.Diagnostics) {
 	tok := p.Peek()
 
diff --git a/json/public.go b/json/public.go
index d3bc9a0..d1e4faf 100644
--- a/json/public.go
+++ b/json/public.go
@@ -71,6 +71,20 @@ func ParseWithStartPos(src []byte, filename string, start hcl.Pos) (*hcl.File, h
 	return file, diags
 }
 
+// ParseExpression parses the given buffer as a standalone JSON expression,
+// returning it as an instance of Expression.
+func ParseExpression(src []byte, filename string) (hcl.Expression, hcl.Diagnostics) {
+	return ParseExpressionWithStartPos(src, filename, hcl.Pos{Byte: 0, Line: 1, Column: 1})
+}
+
+// ParseExpressionWithStartPos parses like json.ParseExpression, but unlike
+// json.ParseExpression you can pass a start position of the given JSON
+// expression as a hcl.Pos.
+func ParseExpressionWithStartPos(src []byte, filename string, start hcl.Pos) (hcl.Expression, hcl.Diagnostics) {
+	node, diags := parseExpression(src, filename, start)
+	return &expression{src: node}, diags
+}
+
 // ParseFile is a convenience wrapper around Parse that first attempts to load
 // data from the given filename, passing the result to Parse if successful.
 //
diff --git a/json/public_test.go b/json/public_test.go
index 70944d9..554b165 100644
--- a/json/public_test.go
+++ b/json/public_test.go
@@ -1,6 +1,7 @@
 package json
 
 import (
+	"fmt"
 	"strings"
 	"testing"
 
@@ -182,3 +183,137 @@ func TestParseWithStartPos(t *testing.T) {
 		t.Errorf("The two ranges did not match: src=%s, part=%s", srcRange, partRange)
 	}
 }
+
+func TestParseExpression(t *testing.T) {
+	tests := []struct {
+		Input string
+		Want  string
+	}{
+		{
+			`"hello"`,
+			`cty.StringVal("hello")`,
+		},
+		{
+			`"hello ${noun}"`,
+			`cty.StringVal("hello world")`,
+		},
+		{
+			"true",
+			"cty.True",
+		},
+		{
+			"false",
+			"cty.False",
+		},
+		{
+			"1",
+			"cty.NumberIntVal(1)",
+		},
+		{
+			"{}",
+			"cty.EmptyObjectVal",
+		},
+		{
+			`{"foo":"bar","baz":1}`,
+			`cty.ObjectVal(map[string]cty.Value{"baz":cty.NumberIntVal(1), "foo":cty.StringVal("bar")})`,
+		},
+		{
+			"[]",
+			"cty.EmptyTupleVal",
+		},
+		{
+			`["1",2,3]`,
+			`cty.TupleVal([]cty.Value{cty.StringVal("1"), cty.NumberIntVal(2), cty.NumberIntVal(3)})`,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.Input, func(t *testing.T) {
+			expr, diags := ParseExpression([]byte(test.Input), "")
+			if diags.HasErrors() {
+				t.Errorf("got %d diagnostics; want 0", len(diags))
+				for _, d := range diags {
+					t.Logf("  - %s", d.Error())
+				}
+			}
+
+			value, diags := expr.Value(&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"noun": cty.StringVal("world"),
+				},
+			})
+			if diags.HasErrors() {
+				t.Errorf("got %d diagnostics on decode value; want 0", len(diags))
+				for _, d := range diags {
+					t.Logf("  - %s", d.Error())
+				}
+			}
+			got := fmt.Sprintf("%#v", value)
+
+			if got != test.Want {
+				t.Errorf("got %s, but want %s", got, test.Want)
+			}
+		})
+	}
+}
+
+func TestParseExpression_malformed(t *testing.T) {
+	src := `invalid`
+	expr, diags := ParseExpression([]byte(src), "")
+	if got, want := len(diags), 1; got != want {
+		t.Errorf("got %d diagnostics; want %d", got, want)
+	}
+	if err, want := diags.Error(), `Invalid JSON keyword`; !strings.Contains(err, want) {
+		t.Errorf("diags are %q, but should contain %q", err, want)
+	}
+	if expr == nil {
+		t.Errorf("got nil Expression; want actual expression")
+	}
+}
+
+func TestParseExpressionWithStartPos(t *testing.T) {
+	src := `{
+  "foo": "bar"
+}`
+	part := `"bar"`
+
+	file, diags := Parse([]byte(src), "")
+	partExpr, partDiags := ParseExpressionWithStartPos([]byte(part), "", hcl.Pos{Byte: 0, Line: 2, Column: 10})
+	if len(diags) != 0 {
+		t.Errorf("got %d diagnostics on parse src; want 0", len(diags))
+		for _, diag := range diags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+	if len(partDiags) != 0 {
+		t.Errorf("got %d diagnostics on parse part src; want 0", len(partDiags))
+		for _, diag := range partDiags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+
+	if file == nil {
+		t.Errorf("got nil File; want actual file")
+	}
+	if file.Body == nil {
+		t.Errorf("got nil Body: want actual body")
+	}
+	if partExpr == nil {
+		t.Errorf("got nil Expression; want actual expression")
+	}
+
+	content, diags := file.Body.Content(&hcl.BodySchema{
+		Attributes: []hcl.AttributeSchema{{Name: "foo"}},
+	})
+	if len(diags) != 0 {
+		t.Errorf("got %d diagnostics on decode; want 0", len(diags))
+		for _, diag := range diags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+	expr := content.Attributes["foo"].Expr
+
+	if expr.Range().String() != partExpr.Range().String() {
+		t.Errorf("The two ranges did not match: src=%s, part=%s", expr.Range(), partExpr.Range())
+	}
+}