How can I implement 2d subscripts via AT-POS for different classes? - raku

Question

here is an MRE (showing two attempts, with debug left in to be helpful) to try and get 2d subscripting working with AT-POS across a DataFrame that has columns of Series...
class Series does Positional {
has Real #.data = [0.1,0.2,0.3];
method AT-POS( $p ) {
#!data[$p]
}
}
class DataFrame does Positional {
has Series #.series;
#`[ ATTEMPT #1
method AT-POS( $p, $q? ) {
given $q {
when Int { #say 'Int';
#!series[$p][$q]
}
when Whatever { #say '*';
#!series[$p].data
}
default { #say 'default';
#!series[$p]
}
}
}
#]
# ATTEMPT #2
method AT-POS(|c) is raw { #`[dd c;] #!series.AT-POS(|c) }
}
my $df = DataFrame.new( series => [Series.new xx 3] );
say $df[1].data; #[0.1 0.2 0.3]
say $df[1][2]; #0.3
say $df[0,1]; #(Series.new(data => $[0.1, 0.2, 0.3]) Series.new(data => $[0.1, 0.2, 0.3]))
say $df[1;2]; #0.3
say $df[1;*]; #got (0.1) ... expected [0.1 0.2 0.3]
say $df[*;1]; #got (0.2) ... wanted [0.2 0.2 0.2]
I already researched on SO and found three related questions here, here and here ... and the attempt #2 in my code seeks to apply #lizmats Answer to the third one. Encouragingly both the attempts in my MRE have the same behaviour. But I cannot workout
why the when Whatever {} option is not entered (attempt #1)
what the |c is doing - even though I can see that it works in the single subscript case (attempt #2)
I have done some experimenting with multi postcircumfix:<[ ]>( DataFrame:D $df, #slicer where Range|List ) is export {} but this seems to overcomplicate matters.
==================
Great answer from #jonathan building on the original from #Lizmat - thanks! Here is the final, working code:
class Series does Positional {
has Real #.data = [0.1,0.2,0.3];
method elems {
#!data.elems
}
method AT-POS( |p ) is raw {
#!data.AT-POS( |p )
}
}
class DataFrame does Positional {
has Series #.series;
method elems {
#!series.elems
}
method AT-POS( |p ) is raw {
#!series.AT-POS( |p )
}
}
my $df = DataFrame.new( series => Series.new xx 3 );
say $df[1].data; #[0.1 0.2 0.3]
say $df[1][2]; #0.3
say $df[0,1]; #(Series.new(data => $[0.1, 0.2, 0.3]) Series.new(data => $[0.1, 0.2, 0.3]))
say $df[1;2]; #0.3
say $df[1;*]; #(0.1 0.2 0.3)
say $df[*;1]; #(0.2 0.2 0.2)

Answer 1

The AT-POS method is only ever passed integer array indices.
The logic to handle slicing (with *, ranges, other iterables, the zen slice) is located in the array indexing operator, which is implemented as the multiple-dispatch subroutine postcircumfix:<[ ]> for single-dimension indexing and postcircumfix:<[; ]> for multi-dimension indexing. The idea is that a class that wants to act as an array-alike need not worry about re-implementing all of the slicing behavior and, further, that the slicing behavior will behave consistently over different user-defined types.
For slicing to work, one must implement elems as well as AT-POS. Adding:
method elems() { #!data.elems }
Is Series and:
method elems() { #!series.elems }
In DataFrame gives the results you're looking for.
If one really wants different slicing semantics, or a far more efficient implementation than the standard one is possible, one can also add multi candidates for the indexing operator (remembering to mark them is export).

Answer 2

This answer is simply an elaboration of the point #raiph made in a comment:
You may be able to simplify the code by using handles
Indeed you can – so much so that I thought it was worth showing what that would look like in a code block without a comment's formatting limits.
Using handles, you can simplify each of the two classes from 9 non-whitespace lines to 3:
class Series does Positional {
has Real #.data handles <elems AT-POS> = [0.1,0.2,0.3];
}
class DataFrame does Positional {
has Series #.series handles <elems AT-POS>;
}
(or you could even have each class on 1 line, if you format them the way I'd be tempted to.)
This code produces all the same results to the say statements from the code in the question.