♻️ Replace the math expressions with own calculator

lib/calculator.dart

@@ -7,6 +7,9 @@ abstract class Expr {
   /// 新增：对表达式进行“求值/数值化”——尽可能把可算的部分算出来
   Expr evaluate();
 
+  /// Substitute variable with value
+  Expr substitute(String varName, Expr value);
+
   @override
   String toString();
 
@@ -27,6 +30,9 @@ class IntExpr extends Expr {
   @override
   Expr evaluate() => this;
 
+  @override
+  Expr substitute(String varName, Expr value) => this;
+
   @override
   String toString() => value.toString();
 }
@@ -42,10 +48,31 @@ class DoubleExpr extends Expr {
   @override
   Expr evaluate() => this;
 
+  @override
+  Expr substitute(String varName, Expr value) => this;
+
   @override
   String toString() => value.toString();
 }
 
+// === VarExpr ===
+class VarExpr extends Expr {
+  final String name;
+  VarExpr(this.name);
+
+  @override
+  Expr simplify() => this;
+
+  @override
+  Expr evaluate() => this;
+
+  @override
+  Expr substitute(String varName, Expr value) => name == varName ? value : this;
+
+  @override
+  String toString() => name;
+}
+
 // === FractionExpr.evaluate ===
 class FractionExpr extends Expr {
   final int numerator;
@@ -74,6 +101,9 @@ class FractionExpr extends Expr {
   @override
   Expr evaluate() => simplify();
 
+  @override
+  Expr substitute(String varName, Expr value) => this;
+
   @override
   String toString() => "$numerator/$denominator";
 }
@@ -150,6 +180,12 @@ class AddExpr extends Expr {
     return AddExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => AddExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left + $right)";
 }
@@ -213,6 +249,12 @@ class SubExpr extends Expr {
     return SubExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => SubExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left - $right)";
 }
@@ -296,6 +338,12 @@ class MulExpr extends Expr {
     return MulExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => MulExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left * $right)";
 }
@@ -378,6 +426,12 @@ class DivExpr extends Expr {
     return DivExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => DivExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left / $right)";
 }
@@ -429,6 +483,10 @@ class SqrtExpr extends Expr {
     return SqrtExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      SqrtExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "sqrt($inner)";
 }
@@ -456,6 +514,10 @@ class CosExpr extends Expr {
     return CosExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      CosExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "cos($inner)";
 }
@@ -483,6 +545,10 @@ class SinExpr extends Expr {
     return SinExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      SinExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "sin($inner)";
 }
@@ -510,6 +576,10 @@ class TanExpr extends Expr {
     return TanExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      TanExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "tan($inner)";
 }