Using » in place of .map is a trap

[AlexDaniel]

There are actually two cases to cover.

I've seen it several times on #perl6 when people tried to do this when they first learned aout »:

<a b c d>».say # Oops!

Well, okay, it works today, but the idea is that the execution order is not defined, so doing ».say is probably not the best idea.

OK, I get it. But still, if I don't care about the execution order, I can use » anywhere I use .map, right?

Oops.

See this:

say (<a b c d>, <x y z>).map: *.elems # (4 3)     ← OK, that's what we want
say (<a b c d>, <x y z>)».elems       # (4 3)      YEAH, it works, great!

# but we may want to use something more
# complex than just a method call. For example this:
say (<a b c d>, <x y z>).map: *.elems × -1     # (-4 -3)
say (<a b c d>, <x y z>)».&(*.elems × -1)      # ((-1 -1 -1 -1) (-1 -1 -1)) – ??? Why?
say (<a b c d>, <x y z>).deepmap: *.elems × -1 # ((-1 -1 -1 -1) (-1 -1 -1)) – OK, so it's like deepmap in this case…

Perhaps see these conversations:

• https://irclog.perlgeek.de/perl6/2016-10-06#i_13343973
• https://irclog.perlgeek.de/perl6/2016-10-08#i_13362495
• https://irclog.perlgeek.de/perl6/2016-10-10#i_13374587

[AlexDaniel]

By the way, we have it mentioned in /operators, but a separate section in traps is not going to hurt.