Factor out expression codegen from function codegen

src/latexify/codegen/__init__.py

@@ -1,5 +1,6 @@
 """Package latexify.codegen."""
 
-from latexify.codegen import function_codegen
+from latexify.codegen import expression_codegen, function_codegen
 
+ExpressionCodegen = expression_codegen.ExpressionCodegen
 FunctionCodegen = function_codegen.FunctionCodegen

src/latexify/codegen/codegen_utils.py

@@ -0,0 +1,28 @@
+from typing import Any
+
+from latexify import exceptions
+
+
+def convert_constant(value: Any) -> str:
+    """Helper to convert constant values to LaTeX.
+
+    Args:
+        value: A constant value.
+
+    Returns:
+        The LaTeX representation of `value`.
+    """
+    if value is None or isinstance(value, bool):
+        return r"\mathrm{" + str(value) + "}"
+    if isinstance(value, (int, float, complex)):
+        # TODO(odashi): Support other symbols for the imaginary unit than j.
+        return str(value)
+    if isinstance(value, str):
+        return r'\textrm{"' + value + '"}'
+    if isinstance(value, bytes):
+        return r"\textrm{" + str(value) + "}"
+    if value is ...:
+        return r"\cdots"
+    raise exceptions.LatexifyNotSupportedError(
+        f"Unrecognized constant: {type(value).__name__}"
+    )