3.5 Evaluate Expression (basic)

cedrickchee · cedrickchee · commit b9d8599e88ec · 2020-03-29T18:42:12.000+08:00
diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
@@ -0,0 +1,46 @@
+package evaluator
+
+// Package evaluator implements the evaluator -- a tree-walker implementation
+// that recursively walks the parsed AST (Abstract Syntax Tree) and evaluates
+// the nodes according to their semantic meaning.
+
+import (
+	"github.com/cedrickchee/hou/ast"
+	"github.com/cedrickchee/hou/object"
+)
+
+// Eval evaluates the node and returns an object.
+func Eval(node ast.Node) object.Object {
+	// Traverse the AST by starting at the top of the tree, receiving an
+	// *ast.Program, and then traverse every node in it.
+
+	switch node := node.(type) {
+
+	// Statements
+	case *ast.Program:
+		// Traverse the tree and evaluate every statement of the *ast.Program.
+		return evalStatements(node.Statements)
+
+	case *ast.ExpressionStatement:
+		// If the statement is an *ast.ExpressionStatement we evaluate its
+		// expression. An expression statement (not a return statement and not
+		// a let statement).
+		return Eval(node.Expression)
+
+	// Expressions
+	case *ast.IntegerLiteral:
+		return &object.Integer{Value: node.Value}
+	}
+
+	return nil
+}
+
+func evalStatements(stmts []ast.Statement) object.Object {
+	var result object.Object
+
+	for _, statement := range stmts {
+		result = Eval(statement)
+	}
+
+	return result
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
@@ -0,0 +1,47 @@
+package evaluator
+
+import (
+	"testing"
+
+	"github.com/cedrickchee/hou/lexer"
+	"github.com/cedrickchee/hou/object"
+	"github.com/cedrickchee/hou/parser"
+)
+
+func TestEvalIntegerExpression(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"5", 5},
+		{"10", 10},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		testIntegerObject(t, evaluated, tt.expected)
+	}
+}
+
+func testEval(input string) object.Object {
+	l := lexer.New(input)
+	p := parser.New(l)
+	program := p.ParseProgram()
+
+	// The heart of the test is the call to Eval.
+	return Eval(program)
+}
+
+func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
+	result, ok := obj.(*object.Integer)
+	if !ok {
+		t.Errorf("object is not Integer. got=%T (%+v)", obj, obj)
+		return false
+	}
+	if result.Value != expected {
+		t.Errorf("object has wrong value. got=%d, want=%d", result.Value, expected)
+		return false
+	}
+
+	return true
+}
