Useless use of "-" in expression "-1" in sink context (line 13)

Question

I'm trying to make tests for a function that throws an exception with this code:

use v6;
use Test;

plan *;

use lib "lib";
use Math::ConvergenceMethods;

sub f ($x) {return $x + 1;}


{
    is-approx: bisection(&f, -2, 0), -1;
    dies-ok: { bisection(&f, 3, 2) }, "Incorrect arguments";
}

done-testing;

And it returns this warnings when I run it:

WARNINGS for /home/antonio/Code/perl6/Math-ConvergenceMethods/t/bisection.t:
Useless use of "-" in expression "-1" in sink context (line 13)
Useless use of constant string "Incorrect arguments" in sink context (lines 14, 14)

How can I fix it?

I think you don't need the colon after `is-approx` (From the Docs https://docs.perl6.org/type/Test#sub_is-approx) — Scimon Proctor, Mar 14 '19 at 11:54
if I delete it returns an error: Two terms in a row across lines (missing semicolon or comma?) — Antonio Gamiz Delgado, Mar 14 '19 at 11:56
oh colon != semicolon... What a fail haha. Thanks for the help :) — Antonio Gamiz Delgado, Mar 14 '19 at 12:01

raiph · Accepted Answer · 2019-03-14T15:06:22.070

The foo in a statement of the form:

foo: ...

is a label where the ... is the statement that it labels.

So the statement you've written is the same as:

bisection(&f, -2, 0), -1;

which leaves the -1 in sink context, hence the error message.

(The message is somewhat LTA because your mistake is clearly that you thought the label syntax was a function calling syntax and while the error message does indicate the error -- in sink context -- the Useless use of "-" is additional detail that doesn't help and probably added to your confusion.)

See also What's the difference these two function calling conventions?.

score 0 · Answer 2 · answered Mar 16 '19 at 03:35

Just want to add for those who might be confused as to when you can call a routine using the colon followed by arguments - it's an alternative format exclusively available when calling methods:

my @list = 1 , 2 , 3 ;
map( { $_ * 2 } , @list );  # "classic" fn call, works with or w/o brackets
@list.map: { $_ * 2 }       # OK  - calling the map method of @list, no brackets or
                            # comma (!) required.
map: { $_ * 2 } , @list ;   # fugetaboutit

Useless use of "-" in expression "-1" in sink context (line 13)

2 Answers2