A pipeline exercise

Run the code below:

CODE:

function f1
{
begin {'begin 1'}
process {$_}
end {'end 1'}
}

function f2
{
begin {'begin 2'}
process {$_}
end {'end 2'}
}

1 | f1 | f2

OUTPUT:

begin 2
begin 1
1
end 1
end 2

Hmm, perhaps at first it looks like the begin block of f2() is executed
before the begin block of f1(). It is not true. Can you figure out what is
actually happening?

Tip: use << Set-PSDebug -trace 1 >> and run it again to see code and
pipeline objects flow.

--
Thanks,
Roman

begin 2
---------
begin 1
1
end 1
---------
end 2


Tip 2: do _not_ use Set-PSDebug to cheat. :P

--
greetings
dreeschkind

And if you use write-host instead of the default formatter, you'll see the
output in the order in which it is processed. Note that the "1" only gets
printed once!

function f1
{
begin {write-host 'begin 1'}
process {write-host $_}
end {write-host 'end 1'}
}

function f2
{
begin {write-host 'begin 2'}
process {write-host $_}
end {write-host 'end 2'}
}

PS> 1 | f1 | f2
begin 1
begin 2
1
end 1
end 2

--
greetings
dreeschkind

"dreeschkind" wrote:

> begin 2
> ---------
> begin 1
> 1
> end 1
> ---------
> end 2
>
>
> Tip 2: do _not_ use Set-PSDebug to cheat. :P
>
> --
> greetings
> dreeschkind

"dreeschkind" wrote:
> ...
> Note that the "1" only gets printed once!

Interesting remark! And yet another puzzle: who writes '1' in this case?

--
Thanks,
Roman

"Roman Kuzmin" wrote:

> "dreeschkind" wrote:
> > ...
> > Note that the "1" only gets printed once!
>
> Interesting remark! And yet another puzzle: who writes '1' in this case?

Since only f1 has input that gets processed in the process block this can
only be done by f1. Write-Host doen't write to the pipeline so f2 has no
input to process.

Btw.: Might be good idea to collect exercises / quizzes / gotchas like this.
This could be included as an interactive tutorial to learn the inner workings
of the shell, thus helping to avoid common mistakes!

--
greetings
dreeschkind

"dreeschkind" wrote:
> Btw.: Might be good idea to collect exercises / quizzes / gotchas like this.
> This could be included as an interactive tutorial to learn the inner workings
> of the shell, thus helping to avoid common mistakes!

This is definitely a good idea. Actually I hope that somebody is already
doing that. So far this forum is the best source but relatively few people
visit it or read everything in it.

--
Thanks,
Roman