p6: Exception Reporting for Sequence operator

p6-x-seq

I am trying to understand how exception reporting (for operator infix:<...>) works 
under different invocations of the perl6 compiler.

Case-1: REPL
---------------------
bash$ perl6
> my @a = 1, 2, 8 ... *
Unable to deduce sequence
>

Case-2: Command-Line 
---------------------------------
bash$ perl6 -e "my @a = 1, 2, 8 ... *"
bash$ 

Case-3: File
-------------
bash$ cat test.p6
my @a = 1, 2, 8 ... *;
bash$ perl6 test.p6
bash$

Observation: Only REPL reports the exception.


Now, let's add some sort of de-reference.

Case-1: REPL
---------------------
bash$ perl6
> my @a = 1, 2, 8 ... *
Unable to deduce sequence
> say @a

> 

Case-2: Command-Line 
---------------------------------
bash$ perl6 -e "my @a = 1, 2, 8 ... *; say @a"
Unable to deduce sequence
  in method sink at src/gen/CORE.setting:10527
  in block  at src/gen/CORE.setting:12592
  in sub coro at src/gen/CORE.setting:5740
  in method reify at src/gen/CORE.setting:5722
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method gimme at src/gen/CORE.setting:5882
  in method join at src/gen/CORE.setting:1293
  in method Str at src/gen/CORE.setting:5793
  in method gist at src/gen/CORE.setting:6099
  in sub say at src/gen/CORE.setting:7602
  in block  at -e:1
bash$ 

Case-3: File
-------------
bash$ cat test.p6
my @a = 1, 2, 8 ... *;
say @a;
bash$ perl6 test.p6
Unable to deduce sequence
  in method sink at src/gen/CORE.setting:10527
  in block  at src/gen/CORE.setting:12592
  in sub coro at src/gen/CORE.setting:5740
  in method reify at src/gen/CORE.setting:5722
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method reify at src/gen/CORE.setting:5492
  in method gimme at src/gen/CORE.setting:5882
  in method join at src/gen/CORE.setting:1293
  in method Str at src/gen/CORE.setting:5793
  in method gist at src/gen/CORE.setting:6099
  in sub say at src/gen/CORE.setting:7602
  in block  at test.p6:2
bash$

Observation: -e and file mode report the exception only on de-reference.
             They also add back trace.
             REPL reports the exception as usual, but no back trace for the de-reference.

This creates the exception:
https://github.com/rakudo/rakudo/blob/nom/src/core/operators.pm#L168

But what triggers different behavior?

timotimo++ explained on #perl6, and this is what I understood:

1. General theory first. Lists are lazy; they are evaluated only on de-reference.
So, if the evaluation were to produce an exception, it would only be at the moment of
the first de-reference.

my @a = 1, 2, 8 ... *; # does NOT produce the exception.
say @a;                # does

2. The REPL prints out the value of the last expression in the line read by it.
https://github.com/perl6/nqp/blob/master/src/HLL/Compiler.pm#L238

> my @a = 1, 2, 8 ... * # Line has only 1 expression.

The print triggers the de-reference. ("say <value>") ... TODO ...