How can I pass a macro to a macro and check for its existence?

Question

I hope this question makes sense. If I wanted to modify the following code to allow the user to pass a macro as the first parameter, how would I do it?

How it is:

(using pgf)

\def\appendtolist#1{% <item>
  \csname pgfutil@ifundefined\endcsname{mylistmacro}{\def\mylistmacro{#1}}{% Instantiate list if not already
  \expandafter\def\expandafter\mylistmacro\expandafter{\mylistmacro^^J#1}}% Append to list
}%

What I effectively want to do:

(using pgf)

\def\appendtolist#1#2{% <macro representing list> <item>
  \csname pgfutil@ifundefined\endcsname{#1}{\def\ithapplist{#1}}{% Instantiate list if not already
  \expandafter\def\expandafter#1\expandafter{\ithapplist^^J#1}}% Append to list
}%

Problem

After \endcsname I need to supply a macro name without the backslash.

Even if I use this other syntax for determining whether or not the macro has been defined, I still run into this problem.

Other csname checks

TeX Kernel

https://tex.stackexchange.com/a/30484/13552

\ifcsname#1\endcsname%
    ... command '#1' exists ...%
  \else%
    ... command '#1' does not exist ...%
  \fi%

LateX kernel

https://tex.stackexchange.com/a/30486/13552emphasized text

\@ifundefined{foo}
  {%
    % \foo not defined
  }
  {%
    % \foo defined
  }%

Ultimate Goal

Normally I spit out things to the log during runtime using \typeout. Instead, I'd like to append them to a macro (delimited variable) that I can call later with something like \typeout\expandafter{\aggregatedlogentries}. This would keep certain things grouped in the log.

Example

Does not compile

 \documentclass{article}
 \usepackage{pgf}
 \def\appendtolist#1{% <item>
       \csname pgfutil@ifundefined\endcsname{mylistmacro}{\def\mylistmacro{#1}}{% Instantiate list if not already
       \expandafter\def\expandafter\mylistmacro\expandafter{\mylistmacro^^J#1}}% Append to list
     }%
\begin{document}
\newcount\step
\step = 0%
\loop
  \appendtolist{\step}% <== I want to be able to provide a macro name as arg1 and item as arg2!
  \advance\step by 1%
  \unless\ifnum \step>10 %space important
\repeat %

\typeout{\mylistmacro}
\end{document}

Answer 1

This is clear example when programmer solves a problem which does not exsist by complicated macro packages (like pgf). You can simply do:

\def\appendtolist#1#2{%
   \ifx#1\undefined \def#1{#2}\else   
   \expandafter\def\expandafter#1\expandafter{#1^^J#2}\fi % Append to list
}%

\appendtolist\ithaplist{aha} \appendtolist\ithaplist{uha}

Answer 2

Your code did not produce an error but simply defined \mylistmacro to be 10 \step as you appended literally the tokens \step^^J 10 times, here I append the expansion of \step's current value, although as wipet rightly notes this is using several orders of magnitude more code than required for the stated use case.

 \documentclass{article}
%no \usepackage{etex}% for \unless
 \usepackage{pgf}
\makeatletter
 \def\appendtolist#1{% <item>
       \pgfutil@ifundefined{mylistmacro}{\def\mylistmacro{#1}}{% Instantiate list if not already
       \expandafter\def\expandafter\mylistmacro\expandafter{\mylistmacro^^J#1}}% Append to list
     }%
\begin{document}
\newcount\step
\step = 0%
\loop
  \expandafter\appendtolist\expandafter{\the\step}% <== I want to be able to provide a macro name as arg1 and item as arg2!
  \advance\step by 1%
  \unless\ifnum \step>10 %space important
\repeat %



\typeout{\mylistmacro}
\end{document}

Answer 3

The LaTeX3 programming layer expl3 provides a lot of macros for these kinds of things.

\documentclass{article}
\usepackage{xparse}
\begin{document}
\ExplSyntaxOn
\seq_new:N \l_macmad_mylist_seq

\int_step_inline:nnnn { 0 } { 1 } { 10 }
 { \seq_put_right:Nn \l_macmad_mylist_seq { #1 } }

\seq_use:Nn \l_macmad_mylist_seq { ,~ }
\ExplSyntaxOff
\end{document}

Answer 4

With etoolbox:

\usepackage{etoolbox}

\newcommand{\appendtolist}[2]{%
  \ifundef{#1}{\gdef#1{== \string#1 ==}}{}%
  \gappto#1{^^J#2}%
}

\appendtolist\mylistmacro{abc}
\appendtolist\mylistmacro{def}
\appendtolist\mylistmacro{ghi}
\appendtolist\mylistmacro{lmn}

\typeout{\mylistmacro}

Output on terminal:

== \mylistmacro ==
abc
def
ghi
lmn

More customizable, because you can define whatever usage you want for the made list, just varying how \showlist is defined.

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\appendtolist}{mm}
 {
  \seq_if_exist:cF { g_macmadness_typeout_#1_seq }
   { \seq_new:c { g_macmadness_typeout_#1_seq } }
  \seq_gput_right:cn { g_macmadness_typeout_#1_seq } { #2 }
 }
\NewDocumentCommand{\showlist}{m}
 {
  \typeout
   {
    ==~#1~==^^J
    \seq_use:cn { g_macmadness_typeout_#1_seq } { ^^J }
   }
 }
\ExplSyntaxOff

\appendtolist{mylistmacro}{abc}
\appendtolist{mylistmacro}{def}
\appendtolist{mylistmacro}{ghi}
\appendtolist{mylistmacro}{lmn}

\showlist{mylistmacro}

Output on terminal

== mylistmacro ==
abc
def
ghi
lmn