fancyhdr and memoir

Question

When I wrote the code below gave the following error:

! LaTeX Error: Command \footruleskip already defined.
               Or name \end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.302 ...and{\footruleskip}{.3\normalbaselineskip}

I want to configure the fancyhdr on memoir.

\documentclass[a4paper]{memoir}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[brazil]{babel}
\usepackage{lipsum}

% fancyhdr on memoir
\usepackage{fancyhdr}
\pagestyle{fancy}

\fancyhead{}
  \renewcommand{\chaptermark}[1]{\markboth{\thechapter.\space#1}{}}
  \renewcommand{\sectionmark}[1]{\markright{\thesection\space#1}}
  \fancyhead[LE]{\bfseries\leftmark}
  \fancyhead[RO]{\bfseries\rightmark}
  \fancyfoot{}
  \fancyfoot[LO,RE]{R\'egis \the\year}
  \fancyfoot[LE,RO]{\bfseries\thepage}
  \renewcommand{\headrulewidth}{0.5pt}
  \renewcommand{\footrulewidth}{0.5pt}
\fancypagestyle{plain}{
  \fancyhead{}
  \renewcommand{\headrulewidth}{0pt}
}

\begin{document}
\pagestyle{fancy}

\chapter{Fancyhdr on Memoir}

\lipsum

\end{document}

Answer 1

memoir defined the command \footruleskip as

\newcommand{\footruleskip}{0.3\normalbaselineskip}

while fancyhdr does the same

\newcommand{\footruleskip}{.3\normalbaselineskip}

(something done since May 7, 1996 with version 1.99 of fancyhdr - a change from a former length \footruleskip to the current control sequence). The good thing is that they're both the same. So, undefining it before loading fancyhdr works:

%...
\let\footruleskip\undefined
\usepackage{fancyhdr}% http://ctan.org/pkg/fancyhdr
%...

Here \undefined is an undefined control sequence that might just as well have been \bleepbloopblah (...as long as that is undefined as well!). See, for example How to undo a \def (i.e., Need a \undef capability) and undefining custom commands.

Although this is your choice, memoir as a solid document class provides ample page header style modifications, and is therefore on the same footing as fancyhdr. Therefore, you may find some conflicting support if using fancyhdr instead of memoir header/footer functionality. Read up on what memoir is capable of in chapter 7 Pagination and headers (p 117) of the memoir documentation. In fact, the documentation mentions (in section 7.2 Page styles, p 118):

These facilities were inspired by the fancyhdr package [..], although the command set is different.