1 files changed, 42 insertions, 22 deletions

diff --git a/mdpoly/algebra.py b/mdpoly/algebra.py
index 6f046b7..d44a528 100644
--- a/mdpoly/algebra.py
+++ b/mdpoly/algebra.py
@@ -1,6 +1,6 @@
 """ Algebraic Structures for Expressions """
 
-from .abc import Expr, Repr
+from .abc import Leaf, Expr, Repr
 from .leaves import Nothing, Const, Param, Var
 from .errors import AlgebraicError
 from .state import State
@@ -16,18 +16,6 @@ from abc import abstractmethod
 import operator
 
 
-def binary_operator(cls: Expr) -> Expr:
-    """ Class decorator to modify constructor of binary operators. """
-    __init_cls__ = cls.__init__
-    @wraps(cls.__init__)
-    def __init_binary__(self, left, right, *args, **kwargs):
-        __init_cls__(self, *args, **kwargs)
-        self.left, self.right = left, right
-
-    cls.__init__ = __init_binary__
-    return cls
-
-
 def unary_operator(cls: Expr) -> Expr:
     """ Class decorator to modify constructor of unary operators. """
     __init_cls__ = cls.__init__
@@ -119,13 +107,45 @@ class ReducibleExpr(HasRepr, Protocol):
         """ Reduce the expression to its basic elements """
 
 
+def binary_operator(left_type: AlgebraicStructure, right_type: AlgebraicStructure):
+    """ Class decorator that adds constructor for binary operations of Expr.
+
+    Classes that inherit :py:class:`mdpoly.abc.Expr` take values for *left* and
+    *right* to represent the operands. This binary operator specifies the
+    algebra (:py:class:`mdpoly.algebra.AlgebraicStructure`) that *left* and
+    *right* have to respect in order for the operation to be correct.
+    Concretely, this is to raise and exception for eg. when a scalar is added
+    to a matrix, etc.
+    """ 
+    # TODO: add right_shape and left_shape for matrices
+    def decorator(cls: Expr) -> Expr:
+        init_cls = cls.__init__
+        @wraps(cls.__init__)
+        def new_init_cls(self, left, right, *args, **kwargs):
+            init_cls(self, *args, **kwargs)
+            # Wrong algebra
+            if not isinstance(left, left_type) or not isinstance(right, right_type):
+                # None of the two is a Leaf. This is a workaround because
+                # adding the algebra to the base Leaf types Const, Var, etc. is
+                # not possible without having circular imports. For the exported types
+                # Constant, Variable, etc. this is not a problem.
+                if not isinstance(left, Leaf) and not isinstance(right, Leaf):
+                    raise AlgebraicError(
+                            "Cannot perform operation between types from "
+                            f"different algebraic structures {type(left)} ({left._algebra}) "
+                            f"and {type(right)} ({right._algebra})")
+
+            self.left, self.right = left, right
+        cls.__init__ = new_init_cls
+        return cls
+    return decorator
+
 # ┏━┓┏━┓╻  ╻ ╻┏┓╻┏━┓┏┳┓╻┏━┓╻     ┏━┓╻  ┏━╸┏━╸┏┓ ┏━┓┏━┓
 # ┣━┛┃ ┃┃  ┗┳┛┃┗┫┃ ┃┃┃┃┃┣━┫┃     ┣━┫┃  ┃╺┓┣╸ ┣┻┓┣┳┛┣━┫
 # ╹  ┗━┛┗━╸ ╹ ╹ ╹┗━┛╹ ╹╹╹ ╹┗━╸   ╹ ╹┗━╸┗━┛┗━╸┗━┛╹┗╸╹ ╹
 
 class PolyRingAlgebra(AlgebraicStructure, Protocol):
     """ Endows with the algebraic structure of a polynomial ring. """
-
     _algebra = AlgebraicStructure.Algebra.poly_ring
     _parameter = Param
     _constant = Const
@@ -172,7 +192,7 @@ class PolyRingAlgebra(AlgebraicStructure, Protocol):
         return Exponentiate(self, other)
 
 
-@binary_operator
+@binary_operator(PolyRingAlgebra, PolyRingAlgebra)
 class Add(Expr, HasRepr, PolyRingAlgebra):
     """ Addition operator between scalar polynomials. """
 
@@ -197,7 +217,7 @@ class Add(Expr, HasRepr, PolyRingAlgebra):
         return f"({self.left} + {self.right})"
 
 
-@binary_operator
+@binary_operator(PolyRingAlgebra, PolyRingAlgebra)
 class Subtract(Expr, ReducibleExpr, PolyRingAlgebra):
     """ Subtraction operator between scalar polynomials. """
 
@@ -209,7 +229,7 @@ class Subtract(Expr, ReducibleExpr, PolyRingAlgebra):
         return f"({self.left} - {self.right})"
 
 
-@binary_operator
+@binary_operator(PolyRingAlgebra, PolyRingAlgebra)
 class Multiply(Expr, HasRepr, PolyRingAlgebra):
     """ Multiplication operator between scalar polynomials. """
 
@@ -235,7 +255,7 @@ class Multiply(Expr, HasRepr, PolyRingAlgebra):
         return f"({self.left} * {self.right})"
 
 
-@binary_operator
+@binary_operator(PolyRingAlgebra, PolyRingAlgebra)
 class Divide(Expr, ReducibleExpr, PolyRingAlgebra):
     """ Division operator between scalar polynomials. """
 
@@ -248,7 +268,7 @@ class Divide(Expr, ReducibleExpr, PolyRingAlgebra):
         return f"({self.left} / {self.right})"
 
 
-@binary_operator
+@binary_operator(PolyRingAlgebra, PolyRingAlgebra)
 class Exponentiate(Expr, ReducibleExpr, PolyRingAlgebra):
     """ Exponentiation operator of scalar polynomials. """
 
@@ -344,16 +364,16 @@ class MatrixAlgebra(AlgebraicStructure, Protocol):
         raise NotImplementedError
 
 
-@binary_operator
+@binary_operator(MatrixAlgebra, MatrixAlgebra)
 class MatAdd(Expr, MatrixAlgebra):
     """ Matrix Addition. """
 
 
-@binary_operator
+@binary_operator(MatrixAlgebra, MatrixAlgebra)
 class ScalarMul(Expr, MatrixAlgebra):
     """ Matrix-Scalar Multiplication. """
 
 
-@binary_operator
+@binary_operator(MatrixAlgebra, MatrixAlgebra)
 class MatMul(Expr, MatrixAlgebra):
     """ Matrix Multiplication. """