fix: Remove ExprIR from Function bases

Resolves (#2572 (comment))

Author: dangotbanned

narwhals/_plan/common.py

@@ -146,9 +146,11 @@ def is_scalar(self) -> bool:
         return False
 
 
-class Function(ExprIR):
+class Function(Immutable):
     """Shared by expr functions and namespace functions.
 
+    Only valid in `FunctionExpr.function`
+
     https://github.com/pola-rs/polars/blob/112cab39380d8bdb82c6b76b31aca9b58c98fd93/crates/polars-plan/src/dsl/expr.rs#L114
     """
 

narwhals/_plan/expr.py

@@ -180,6 +180,10 @@ class FunctionExpr(ExprIR, t.Generic[_FunctionT]):
     2. The union of (1) and any `FunctionOptions` in `inputs`
     """
 
+    @property
+    def is_scalar(self) -> bool:
+        return self.function.is_scalar
+
     def with_options(self, options: FunctionOptions, /) -> Self:
         options = self.options.with_flags(options.flags)
         return type(self)(input=self.input, function=self.function, options=options)