[FEAT] Add getter for Struct and List expressions

[kevinzwang]

This PR adds getters for both lists and structs, which enables the access of elements in ListArray, FixedSizeListArray, and StructArray types.

For lists:

Expression.list.get(idx, default=None)

This method takes in an index, which can be a single value or an expression, and optionally a default value, which will be set when the index is out of bounds. It retrieves the value at index of each list in the array.

For structs:

Expression.struct.get(name)

This method takes in the name of the field to get, and returns an expression of that struct's field.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a994c7b) 84.60% compared to head (e945aca) 84.88%.
Report is 6 commits behind head on main.

❗ Current head e945aca differs from pull request most recent head 47fa923. Consider uploading reports for the commit 47fa923 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1775      +/-   ##
==========================================
+ Coverage   84.60%   84.88%   +0.28%     
==========================================
  Files          55       55              
  Lines        5554     5604      +50     
==========================================
+ Hits         4699     4757      +58     
+ Misses        855      847       -8     

Files	Coverage Δ
daft/expressions/expressions.py	92.37% <100.00%> (+0.28%)	⬆️
daft/series.py	92.50% <100.00%> (-0.13%)	⬇️

... and 8 files with indirect coverage changes

[kevinzwang]

default isn't a reserved keyword in Rust but the syntax highlighting is different on it. Maybe because it's in std.

[jaychia]

Looking good so far!

[jaychia]

I think we can probably make an assertion that default has length 1 and only allow for that case from the user-API.

Should be pretty rare that a user wants to have a different default per-element!

[jaychia]

In Python-land you can wrap the default into a LiteralExpression like so:

def get(idx: int, default: Any):
    default_expr = lit(default)
    ...

Then when it gets evaluated in Rust it should evaluate the a Series with length 1 I believe (we should double-check!)

[kevinzwang]

Thanks! This method does seem to work, although it doesn't seem to emit any errors when you pass in an Expression, it just manages it as a generic PyObject.

I think this is probably better handled on the Rust side, but is there a way to check of an expression's type is castable to another's?

[jaychia]

Looking good :)

[kevinzwang]

Sorry that this PR is getting a little larger than expected now, most of the changed files are just adding implementations for with_validity() and validity() which are relatively straightforward.

[jaychia]

Great work!

Let's also add these expressions into our documentation: https://github.com/Eventual-Inc/Daft/blob/main/docs/source/api_docs/expressions.rst#nested