chore: clean up impl

ibis/backends/sql/compilers/clickhouse.py

@@ -2,7 +2,6 @@
 
 import calendar
 import math
-from functools import partial
 from string import whitespace
 from typing import TYPE_CHECKING, Any
 
@@ -378,18 +377,22 @@ def visit_TimestampFromUNIX(self, op, *, arg, unit):
 
     def visit_TimestampTruncate(self, op, *, arg, unit):
         if (short := unit.short) == "W":
-            func = "toMonday"
+            funcname = "toMonday"
         else:
-            func = f"toStartOf{unit.singular.capitalize()}"
+            funcname = f"toStartOf{unit.singular.capitalize()}"
+
+        func = self.f[funcname]
 
         if short in ("Y", "Q", "M", "W", "D"):
-            cast = partial(self.cast, to=op.dtype)
-        else:
-            cast = lambda x: x
+            # these units return `Date` so we have to cast back to the
+            # corresponding Ibis type
+            return self.cast(func(arg), op.dtype)
 
-        if short in ("s", "ms", "us", "ns"):
-            arg = self.f.toDateTime64(arg, op.arg.dtype.scale or 0)
-        return cast(self.f[func](arg))
+        elif short in ("s", "ms", "us", "ns"):
+            return func(self.f.toDateTime64(arg, op.arg.dtype.scale or 0))
+        else:
+            assert short in ("h", "m"), short
+            return func(arg)
 
     visit_TimeTruncate = visit_TimestampTruncate