Implement HasRepr for Param and fix repr of Const

2 files changed, 17 insertions, 4 deletions

diff --git a/mdpoly/errors.py b/mdpoly/errors.py
index 1531a84..578c5c8 100644
--- a/mdpoly/errors.py
+++ b/mdpoly/errors.py
@@ -9,3 +9,8 @@ class InvalidShape(Exception):
     """
     This is raised whenever there is an invalid shape
     """
+
+class MissingParameters(Exception):
+    """
+    This is raised whenever
+    """
diff --git a/mdpoly/leaves.py b/mdpoly/leaves.py
index 8a018e6..a9d2005 100644
--- a/mdpoly/leaves.py
+++ b/mdpoly/leaves.py
@@ -1,6 +1,7 @@
 from .abc import Leaf, Repr
 from .types import Number, Shape, MatrixIndex, PolyVarIndex, PolyIndex
 from .state import State
+from .errors import MissingParameters
 from .representations import HasRepr
 
 from typing import TypeVar
@@ -33,10 +34,10 @@ class Const(Leaf, HasRepr):
         return r, state
 
     def __repr__(self) -> str:
-        if self.name:
-            return self.name
+        if not self.name:
+            return str(self.value)
 
-        return repr(self.value)
+        return self.name
 
 
 @dataclass(frozen=True)
@@ -58,12 +59,19 @@ class Var(Leaf, HasRepr):
 
 
 @dataclass(frozen=True)
-class Param(Leaf):
+class Param(Leaf, HasRepr):
     """
     A parameter
     """
     name: str
     shape: Shape = Shape.scalar
 
+    def to_repr(self, repr_type: type[T], state: State) -> T:
+        if self not in state.parameters:
+            raise MissingParameters("Cannot construct representation because "
+                                    f"value for parameter {self} was not given.")
+
+        return Const(state.parameters[self]).to_repr(repr_type, state)
+
     def __repr__(self) -> str:
         return self.name