basic implementation of DisplayForm

CHANGES.rst

@@ -15,6 +15,7 @@ New Builtins
 #. ``Accuracy``
 #. ``ClebschGordan``
 #. ``Curl`` (2-D and 3-D vector forms only)
+#. ``DisplayForm``
 #. ``Kurtosis``
 #. ``PauliMatrix``
 #. ``Remove``
@@ -57,7 +58,8 @@ Enhancements
 #. Better handling of comparisons with finite precision numbers.
 #. Improved implementation for  ``Precision``.
 #. Infix operators, like ``->`` render with their Unicode symbol when ``$CharacterEncoding`` is not "ASCII".
-
+#. Basic implementation of ``DisplayForm``
+
 5.0.2
 -----
 

mathics/builtin/forms/output.py

@@ -149,6 +149,50 @@ def apply_makeboxes(self, expr, n, f, evaluation):
             )
 
 
+class DisplayForm(FormBaseClass):
+    """
+    <dl>
+      <dt>'DisplayForm[$obj$]'
+      <dd>is a form that shows low-level box structures as the correspoding
+      print form.
+    </dl>
+
+    By default, box structures are shown without a format:
+    >> box = MakeBoxes[Infix[{a, b}," + "], StandardForm]
+     = RowBox[{a,  + , b}]
+    Wrapping the box structure in a $DisplayForm$ ensures to render the expression:
+    >> DisplayForm[box]
+     = a + b
+
+    The same is valid if the box is nested into an expression:
+    >> DisplayForm[F[box]]
+     = F[a + b]
+
+    If the argument does not have a box structure,  `DisplayForm` is ignored:
+    >> DisplayForm[a + b]
+     = a + b
+
+    """
+
+    in_outputforms = True
+    summary_text = "render low-level box structures"
+
+    def apply_makeboxes(self, expr, f, evaluation):
+        """MakeBoxes[DisplayForm[expr_], f_]"""
+
+        # This is a paratemer needed to remember if
+        # MakeBoxes is formatting an expression inside a DisplayForm.
+        # Hopefully, this is temporal and it is not going to be
+        # needed after the Format/MakeBoxes refactor.
+        evaluation.in_display_form = True
+        try:
+            result = MakeBoxes(expr, f).evaluate(evaluation)
+        except:
+            pass
+        evaluation.in_display_form = False
+        return result
+
+
 class FullForm(FormBaseClass):
     """
     <url>

mathics/builtin/makeboxes.py

@@ -394,6 +394,10 @@ def apply_general(self, expr, f, evaluation):
         """MakeBoxes[expr_,
         f:TraditionalForm|StandardForm|OutputForm|InputForm|FullForm]"""
         if isinstance(expr, BoxElementMixin):
+            # If we are inside a DisplayForm block,
+            # BoxElementMixin are not processed.
+            if evaluation.in_display_form:
+                return expr
             expr = expr.to_expression()
         if isinstance(expr, Atom):
             return expr.atom_to_boxes(f, evaluation)

mathics/core/evaluation.py

@@ -256,6 +256,9 @@ def __init__(
 
         self.quiet_all = False
         self.format = format
+
+        # See comment in mathics.builtin.makeboxes.DisplayForm
+        self.in_display_form = False
         self.catch_interrupt = catch_interrupt
         self.SymbolNull = SymbolNull