Pass an operator like a function

Question

While learning Raku, I arrive to the point of higher order function and the sort function.

I have this example:

> sort <4 6 2 9 1 5 11>
(1 2 4 5 6 9 11)

Then the doc for the routine says this:

Sorts the list, smallest element first. By default infix:<cmp>  is used for
comparing list elements.

And the book that I'm following "Piensa en raku", in section 9.2, makes a comparative between numeric sort and lexicographic sort.

I tried the following:

> sort &le, <4 6 2 9 1 5 11>;
===SORRY!=== Error while compiling:
Undeclared routine:
    le used at line 1. Did you mean 'lc'?

But getting this problem, instead of the sorted list lexicographically ordered. So may be is too early for me to understand this, but should be possible to pass an operator in Raku like a function, since also the documentation says that it is using an infix: or I need to do something like this, maybe I'm confusing operators and subroutines:

sub my-le($a,$b) {
    $a le $b;
}

sort &my-le, <4 6 2 9 1 5 11>;

or this:

   sort { $^b le $^a  }, <4 6 2 9 1 5 11>;

So I have the question about different use the infix operator and a subroutine, maybe the problem about this difference is that the order of the operands affect the result of some operations. so you can not use so lightly passing it as a function or a parameter or variable

> sort { $^a le $^b  }, <4 6 2 9 1 5 11>
(9 6 5 4 2 11 1)
> sort { $^b le $^a  }, <4 6 2 9 1 5 11>
(1 11 2 4 5 6 9)

I hope that I can explained my doubts on this problem.

score 19 · Accepted Answer · answered Oct 13 '20 at 09:59

The le operator is defined something like this :

sub infix:<le> { ... }

The infix: prefix tells the language it's a infix operator. So if you want to call is as a sub its &infix:<le>(1,2) or for your sort :

sort &infix:<le>, <4 6 2 9 1 5 11>;
(9 6 5 4 2 11 1)

This may help https://docs.raku.org/language/optut

jakar · Answer 2 · 2020-10-13T11:03:30.810

12

Adding to Scimon's answer and in case you want a more "clear" presentation, you can use the following whatever construct:

sort *le*, <4 6 2 9 1 5 11>;

The previous code is a whatever construct (code object).

The whatever construct uses multiple * to generate blocks of code (anonymous subroutines) with as many arguments:

my $c = * + *;   # same as   -> $a, $b { $a + $b }

This means that the whatever sign (*) is used to denote the two (ore more) parameters that are used in the calculation

The same goes for our case:

sort *le*, <4 6 2 9 1 5 11>;

is the same as :

sort * le *, <4 6 2 9 1 5 11>;   # adding space between operator and the whatever sign (*)

and this by turn is the same as :

sort -> $a, $b { $a le $b }, <4 6 2 9 1 5 11>;   # anonymous subroutine

or

sort { $^a le $^b }, <4 6 2 9 1 5 11>;

or even

sub le {
    $^a le $^b
}

sort &le, <4 6 2 9 1 5 11>;

References:

https://docs.raku.org/type/Whatever

https://docs.raku.org/type/Block

edited Oct 13 '20 at 11:03

answered Oct 13 '20 at 10:12

jakar

1,701
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2

FWIW, I would write that with additional whitespace, for clarity: `sort * le *, <4 6 2 9 1 5 11>`. Otherwise one might think that `*le*` itself is a thing. – Elizabeth Mattijsen Oct 13 '20 at 10:18
1

I wrote that deliberately so, to resemble kind of a circumfix operator. But of course you are right !! – jakar Oct 13 '20 at 10:22
1

You might want to add an explanation of what this is doing. How `*le*` is turned into `{$^a le $^b}`. – Scimon Proctor Oct 13 '20 at 10:24
2

That's a great update and a perfect example of TIMTOWDI in action. Both options are equally valid. – Scimon Proctor Oct 13 '20 at 12:33

Pass an operator like a function

2 Answers2