Support to_sympy for If

mathics/builtin/procedural.py

@@ -22,7 +22,7 @@
     A_PROTECTED,
     A_READ_PROTECTED,
 )
-from mathics.core.builtin import Builtin, InfixOperator, IterationFunction
+from mathics.core.builtin import Builtin, InfixOperator, IterationFunction, SympyObject
 from mathics.core.evaluation import Evaluation
 from mathics.core.expression import Expression
 from mathics.core.interrupt import (
@@ -37,6 +37,8 @@
 from mathics.eval.datetime import eval_pause, valid_time_from_expression
 from mathics.eval.patterns import match
 
+import sympy
+
 SymbolWhich = Symbol("Which")
 
 
@@ -365,7 +367,8 @@ def eval(self, start, test, incr, body, evaluation):
         return SymbolNull
 
 
-class If(Builtin):
+# note that this is not strictly speaking procedural
+class If(SympyObject):
     """
     <url>:WMA link:https://reference.wolfram.com/language/ref/If.html</url>
 
@@ -443,6 +446,21 @@ def eval_with_false_and_other(self, condition, t, f, u, evaluation):
         else:
             return u.evaluate(evaluation)
 
+    def to_sympy(self, expr, **kwargs):
+        args = [e.to_sympy() for e in expr.elements]
+        if all(arg is not None for arg in args):
+            if len(args) == 2:
+                cond, t = args
+                return sympy.Piecewise((t, cond))
+            elif len(args) == 3:
+                cond, t, f = args
+                return sympy.Piecewise((t, cond), (f, True))
+            elif len(args) == 4:
+                cond, t, f, u = args
+                return sympy.Piecewise((t, cond==True), (f, cond==False), (u, True))
+
+        return None
+
 
 class Interrupt(Builtin):
     r"""
@@ -542,6 +560,7 @@ def eval(self, expr, evaluation: Evaluation):  # pylint: disable=unused-argument
         raise ReturnInterrupt(expr)
 
 
+# TODO: to_sympy like If
 class Switch(Builtin):
     """
     <url>:WMA link:
@@ -648,6 +667,7 @@ def eval_with_tag(
         raise WLThrowInterrupt(value, tag)
 
 
+# TODO: to_sympy like If
 class Which(Builtin):
     """
     <url>