How can a command take #1 as an argument?

Question

I was wondering how some commands manage to take #1 as an argument when used, not when declared.

For instance, biblatex's \DeclareFieldFormat can be used like this:

\DeclareFieldFormat {postnote} {#1}

I have taken a look at biblatex1.sty (lines 3480--3506, v2.3, revison 28161) but couldn't figure out how it can do that. Where is the trick?

This is what I want to achieve:

\foo {bar} {\textbf{#1}}

I want it to typeset bar in bold face; that is, to typeset whatever is given in the first argument in whatever format is given in the second one.

From biblatex1.sty:

%---CUT---
\newrobustcmd*{\DeclareFieldFormat}{%
  \@ifstar
    {\blx@defformat\blx@defplainformat{ffd}*}
    {\blx@defformat\blx@defplainformat{ffd}{}}}
%---CUT---

% {<macro>}{<class>}{<*>}

\def\blx@defformat#1#2#3{%
  \@ifnextchar[%]
    {\blx@defformat@i{#1}{#2}{#3}}
    {\blx@defformat@i{#1}{#2}{#3}[*]}}

% {<macro>}{<class>}{<*>}[<entrytype>]{<name>}

\def\blx@defformat@i#1#2#3[#4]#5{%
  \notblank{#3}
    {\blx@resetformat{#2}{#5}}
    {}%
  \def\blx@defformat@a{#2}%
  \def\blx@defformat@b{#4}%
  \blx@xsanitizeafter{\def\blx@defformat@c}{#5}%
  \afterassignment\blx@defformat@ii
  #1}
%---CUT---

Answer 1

You need an auxiliary function, which is defined with one argument:

\documentclass{article}
\newcommand{\foo}[2]{%
  \def\fooaux##1{#2}%
  \fooaux{#1}%
}
\newcommand{\fooaux}[1]{}
\begin{document}
\foo{bar}{\textbf{#1,}} hhhh
\end{document}

The idea here is that \fooaux will end up defined as equivalent to

\renewcommand{\fooaux}[1]{%
  \textbf{#1,}%
}

in the code above.