agg_concat doesn't work on strings

[Vince7778]

The docs seem to claim that agg_concat works on strings, when in fact it only works on List and Python types. A workaround is doing agg_list() then .list.join(), but we should really add support for agg_concat directly on strings.

In [35]: df3 = daft.from_pydict({"a": ["the", " quick", " brown", " fox"]})
In [36]: df3.agg(col("a").agg_concat()).show()
---------------------------------------------------------------------------
DaftCoreException                         Traceback (most recent call last)
Cell In[36], line 1
----> 1 df3.agg(col("a").agg_concat()).show()

File ~/Documents/Programming/daft/Daft/daft/api_annotations.py:26, in DataframePublicAPI.<locals>._wrap(*args, **kwargs)
     24 type_check_function(func, *args, **kwargs)
     25 timed_method = time_df_method(func)
---> 26 return timed_method(*args, **kwargs)

File ~/Documents/Programming/daft/Daft/daft/analytics.py:198, in time_df_method.<locals>.tracked_method(*args, **kwargs)
    195 @functools.wraps(method)
    196 def tracked_method(*args, **kwargs):
    197     if _ANALYTICS_CLIENT is None:
--> 198         return method(*args, **kwargs)
    200     start = time.time()
    201     try:

File ~/Documents/Programming/daft/Daft/daft/dataframe/dataframe.py:2226, in DataFrame.agg(self, *to_agg)
   2223     if not isinstance(expr, Expression):
   2224         raise ValueError(f"DataFrame.agg() only accepts expression type, received: {type(expr)}")
-> 2226 return self._agg(to_agg_list, group_by=None)

File ~/Documents/Programming/daft/Daft/daft/dataframe/dataframe.py:1996, in DataFrame._agg(self, to_agg, group_by)
   1991 def _agg(
   1992     self,
   1993     to_agg: Iterable[Expression],
   1994     group_by: Optional[ExpressionsProjection] = None,
   1995 ) -> "DataFrame":
-> 1996     builder = self._builder.agg(list(to_agg), list(group_by) if group_by is not None else None)
   1997     return DataFrame(builder)

File ~/Documents/Programming/daft/Daft/daft/logical/builder.py:227, in LogicalPlanBuilder.agg(self, to_agg, group_by)
    221 def agg(
    222     self,
    223     to_agg: list[Expression],
    224     group_by: list[Expression] | None,
    225 ) -> LogicalPlanBuilder:
    226     group_by_pyexprs = [expr._expr for expr in group_by] if group_by is not None else []
--> 227     builder = self._builder.aggregate([expr._expr for expr in to_agg], group_by_pyexprs)
    228     return LogicalPlanBuilder(builder)

DaftCoreException: DaftError::External Unable to create logical plan node.
Due to: DaftError::TypeError We can only perform List Concat Agg on List or Python Types, got dtype Utf8 for column "a"

[vicky1999]

@Vince7778 Can I work on this issue?

[samster25]

@vicky1999 assigning the issue to you!

[colin-ho]

Closed with #2847