On 2020-05-05 William Michels via perl6-users <[email protected]>
wrote:
> If the only difference between the "-n" and "-p" flags is really that
> the second one autoprints $_, I would have expected the "-pe" code
> above to work identically to the "-ne" case (except "-ne" requires a
> print, put or say). Presumably 'perl6 -ne "put .chop" ' is the same
> as 'perl6 -ne ".chop.put" ' , so if ".put" isn't returning $_ ,
> what is it returning then?
Ok, let's ask rakudo what the difference is between the two programs::
$ diff -uw <(rakudo --target=ast -ne '.chop') \
<(rakudo --target=ast -pe '.chop')
--- /dev/fd/63 2020-05-05 20:36:57.262256451 +0100
+++ /dev/fd/62 2020-05-05 20:36:57.263256462 +0100
@@ -78,6 +78,7 @@
- QAST::Op(p6scalarfromdesc)
- QAST::WVal(ContainerDescriptor::Untyped)
- QAST::Var(local __lowered_param_decont__1)
+ - QAST::Stmts
- QAST::Stmts <sunk> .chop
- QAST::Stmt <sunk final> .chop
- QAST::Want <sunk>
@@ -90,6 +91,8 @@
- QAST::Op(hllize) <sunk> :statement_id<1>
- QAST::Op(callmethod chop) chop
- QAST::Var(lexical $_) <wanted>
+ - QAST::Op(call &say)
+ - QAST::Var(lexical $_)
- QAST::Stmts
- QAST::Op(bind)
- QAST::Var(local ctxsave :decl(var))
Which tells us that the only difference is (as documented) that ``-p``
adds a ``say $_`` at the end of the body of the loop.
So, ``-ne 'put .chop'`` is roughly equivalent to::
for $*ARGFILES.lines { put .chop }
Here ``.chop`` (or its fully explicit version, ``$_.chop``) returns
the chopped line, which gets passed to ``put``, which prints it.
On the other had, ``-pe '.chop'`` is roughly equivalent to::
for $*ARGFILES.lines { .chop; say $_ }
Here, ``.chop`` still returns the chopped line, but that value is
thrown away. Then, independently of that, we print the value of
``$_``.
In both cases, nothing is changing the value of ``$_`` inside each
iteration of the loop.
The two programs behave differently because they are not the same
program! Two pairs of equivalent programs would be::
raku -pe '.chop'
raku -ne '.chop; .say'
raku -pe '.=chop'
raku -ne '.=chop; .say'
Does this help?
--
Dakkar - <Mobilis in mobile>
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