Add view kwarg to first and last

[Chandu-4444]

Please look at this, I've added view (defaults to false) kwarg to first and last. Also added tests to check for checking types of the results that were returned by the above functions. #2845

[nalimilan]

It would be good to have an @inferred test if that's doable.

[bkamins]

It would be good to have an @inferred test if that's doable.

My experience is that it will be problematic, but we can check :).

[Chandu-4444]

I didn't clearly understand the working of @inferred. Is it possible to point a few resources, I tried this https://docs.julialang.org/en/v1/stdlib/Test/#Test.@inferred, but still I wasn't very obvious about this.

[bkamins]

@inferred checks if Julia compiler is able to determine the return value of the function.

In our case here the question is if compiler is able to determine that when you do not pass view kwarg the returned value will have DataFrame type.

So just check if @inferred first(df, 6) errors or not. If it errors it is not the end of the world. For example with filter we have:

julia> df = DataFrame(x=1:3)
3×1 DataFrame
 Row │ x
     │ Int64
─────┼───────
   1 │     1
   2 │     2
   3 │     3

julia> @inferred filter(x -> true, df)
3×1 DataFrame
 Row │ x
     │ Int64
─────┼───────
   1 │     1
   2 │     2
   3 │     3

julia> @inferred filter(x -> true, df, view=false)
ERROR: return type DataFrame does not match inferred return type Union{DataFrame, SubDataFrame{DataFrame, DataFrames.Index, T} where T<:AbstractVector{Int64}}

and both results are expected and optimal. By default compiler knows that the returned value will have DataFrame type, however, when you pass view kwarg the return type is a union as at compile time the value of view kwarg cannot be determined.

[Chandu-4444]

So just check if @inferred first(df, 6) errors or not.

Yes, that throws an error.

[bkamins]

I have pushed changes that should make it @inferred hopefully.

[bkamins]

Yes - my fix works.

[bkamins]

Looks good (subject to wording of docstrings, which @nalimilan is better at checking than me)

[nalimilan]

Thanks!

[bkamins]

Thank you!