Dry up incremental expression construction from constants, variable, terms and other expressions

packages/cassowary/lib/constant_member.dart

@@ -8,57 +8,7 @@ class ConstantMember extends EquationMember {
   double value = 0.0;
   ConstantMember(this.value);
 
-  Expression _asExpression() => new Expression([], this.value);
-
-  Constraint operator >=(EquationMember m) => _asExpression() >= m;
-
-  Constraint operator <=(EquationMember m) => _asExpression() <= m;
-
-  operator ==(EquationMember m) => _asExpression() == m;
-
-  Expression operator +(EquationMember m) {
-    if (m is ConstantMember) {
-      return new Expression([], this.value + m.value);
-    }
-
-    if (m is Variable) {
-      return new Expression([new Term(m, 1.0)], this.value);
-    }
-
-    if (m is Term) {
-      return new Expression([m], this.value);
-    }
-
-    if (m is Expression) {
-      return new Expression(new List.from(m.terms), this.value + m.constant);
-    }
-
-    assert(false);
-    return null;
-  }
-
-  Expression operator -(EquationMember m) {
-    if (m is ConstantMember) {
-      return new Expression([], this.value - m.value);
-    }
-
-    if (m is Variable) {
-      return new Expression([new Term(m, -1.0)], this.value);
-    }
-
-    if (m is Term) {
-      return new Expression([new Term(m.variable, -m.coefficient)], this.value);
-    }
-
-    if (m is Expression) {
-      var negatedTerms = m.terms.fold(new List<Term>(), (list, term) => list
-        ..add(new Term(term.variable, -term.coefficient)));
-      return new Expression(negatedTerms, this.value - m.constant);
-    }
-
-    assert(false);
-    return null;
-  }
+  Expression asExpression() => new Expression([], this.value);
 
   EquationMember operator *(double m) {
     return new ConstantMember(this.value * m);

packages/cassowary/lib/equation_member.dart

@@ -4,4 +4,16 @@
 
 part of cassowary;
 
-abstract class EquationMember {}
+abstract class EquationMember {
+  Expression asExpression();
+
+  Constraint operator >=(EquationMember m) => asExpression() >= m;
+
+  Constraint operator <=(EquationMember m) => asExpression() <= m;
+
+  operator ==(EquationMember m) => asExpression() == m;
+
+  Expression operator +(EquationMember m) => asExpression() + m;
+
+  Expression operator -(EquationMember m) => asExpression() - m;
+}

packages/cassowary/lib/expression.dart

@@ -12,6 +12,8 @@ class Expression extends EquationMember {
 
   Expression(this.terms, this.constant);
 
+  Expression asExpression() => this;
+
   Constraint _createConstraint(
       EquationMember /* rhs */ value, Relation relation) {
     if (value is ConstantMember) {

packages/cassowary/lib/term.dart

@@ -11,60 +11,9 @@ class Term extends EquationMember {
 
   Term(this.variable, this.coefficient);
 
-  Expression _asExpression() =>
+  Expression asExpression() =>
       new Expression([new Term(this.variable, this.coefficient)], 0.0);
 
-  Constraint operator >=(EquationMember m) => _asExpression() >= m;
-
-  Constraint operator <=(EquationMember m) => _asExpression() <= m;
-
-  operator ==(EquationMember m) => _asExpression() == m;
-
-  Expression operator +(EquationMember m) {
-    if (m is ConstantMember) {
-      return new Expression([this], m.value);
-    }
-
-    if (m is Variable) {
-      return new Expression([this, new Term(m, 1.0)], 0.0);
-    }
-
-    if (m is Term) {
-      return new Expression([this, m], 0.0);
-    }
-
-    if (m is Expression) {
-      return new Expression(
-          new List.from(m.terms)..insert(0, this), m.constant);
-    }
-
-    assert(false);
-    return null;
-  }
-
-  Expression operator -(EquationMember m) {
-    if (m is ConstantMember) {
-      return new Expression([this], -m.value);
-    }
-
-    if (m is Variable) {
-      return new Expression([this, new Term(m, -1.0)], 0.0);
-    }
-
-    if (m is Term) {
-      return new Expression([this, new Term(m.variable, -m.coefficient)], 0.0);
-    }
-
-    if (m is Expression) {
-      var negatedTerms = m.terms.fold(new List<Term>(),
-          (list, t) => list..add(new Term(t.variable, -t.coefficient)));
-      return new Expression(negatedTerms..insert(0, this), -m.constant);
-    }
-
-    assert(false);
-    return null;
-  }
-
   EquationMember operator *(double m) {
     return new Term(this.variable, this.coefficient * m);
   }

packages/cassowary/lib/variable.dart

@@ -8,60 +8,7 @@ class Variable extends EquationMember {
   double value = 0.0;
   Variable(this.value);
 
-  Expression _asExpression() => new Expression([new Term(this, 1.0)], 0.0);
-
-  Constraint operator >=(EquationMember m) => _asExpression() >= m;
-
-  Constraint operator <=(EquationMember m) => _asExpression() <= m;
-
-  operator ==(EquationMember m) => _asExpression() == m;
-
-  Expression operator +(EquationMember m) {
-    if (m is ConstantMember) {
-      return new Expression([new Term(this, 1.0)], m.value);
-    }
-
-    if (m is Variable) {
-      return new Expression([new Term(this, 1.0), new Term(m, 1.0)], 0.0);
-    }
-
-    if (m is Term) {
-      return new Expression([new Term(this, 1.0), m], 0.0);
-    }
-
-    if (m is Expression) {
-      return new Expression(
-          new List.from(m.terms)..insert(0, new Term(this, 1.0)), m.constant);
-    }
-
-    assert(false);
-    return null;
-  }
-
-  Expression operator -(EquationMember m) {
-    if (m is ConstantMember) {
-      return new Expression([new Term(this, 1.0)], -m.value);
-    }
-
-    if (m is Variable) {
-      return new Expression([new Term(this, 1.0), new Term(m, -1.0)], 0.0);
-    }
-
-    if (m is Term) {
-      return new Expression(
-          [new Term(this, 1.0), new Term(m.variable, -m.coefficient)], 0.0);
-    }
-
-    if (m is Expression) {
-      var negatedTerms = m.terms.fold(new List<Term>(),
-          (list, t) => list..add(new Term(t.variable, -t.coefficient)));
-      negatedTerms.insert(0, new Term(this, 1.0));
-      return new Expression(negatedTerms, -m.constant);
-    }
-
-    assert(false);
-    return null;
-  }
+  Expression asExpression() => new Expression([new Term(this, 1.0)], 0.0);
 
   EquationMember operator *(double m) {
     return new Term(this, m);