Wrapping existing bibmacro
There is no \letbibmacro
(and you might file a feature request), but it's not difficult to create one:
\documentclass{article}
\usepackage[style=authoryear-comp]{biblatex}
\usepackage{hyperref}
\newcommand{\letbibmacro}[2]{%
\csletcs{abx@macro@#1}{abx@macro@#2}%
}
\letbibmacro{orig-url+urldate}{url+urldate}
\renewbibmacro*{url+urldate}{%
\iffieldundef{doi}
{\usebibmacro{orig-url+urldate}}
{}%
}
\addbibresource{\jobname.bib}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{key,
author = {Author, A.},
year = {2001},
title = {Title},
publisher = {Publisher},
archivePrefix = {arXiv},
arxivId = {1607.07249},
doi = {10.1007/978-3-319-49004-5_22},
eprint = {1607.07249},
url = {http://link.springer.com/10.1007/978-3-319-49004-5{\_}22},
}
@book{key2,
author = {Author, A.},
year = {2001},
title = {Title},
publisher = {Publisher},
archivePrefix = {arXiv},
arxivId = {1607.07249},
eprint = {1607.07249},
url = {http://link.springer.com/10.1007/978-3-319-49004-5{\_}22},
}
\end{filecontents}
\begin{document}
\nocite{*}
\printbibliography
\end{document}
You have already gotten great answers to the question you asked, so I thought I'd add two solutions for the problem that you want to solve.
You can use
\DeclareSourcemap{
\maps[datatype=bibtex]{
\map{
\step[fieldsource=doi, final]
\step[fieldset=url, null]
\step[fieldset=urldate, null]
}
}
}
or
\AtEveryBibitem{%
\iffieldundef{doi}
{}
{\clearfield{url}\clearfield{urlyear}}%
}
to not print the URL if a DOI is present.
To conditionally print the url, you could wrap it in \iffieldundef{doi}{}{}
\documentclass{article}
\usepackage[style=authoryear-comp]{biblatex}
\usepackage{hyperref}
% print url if no doi
\renewbibmacro*{doi+eprint+url}{%
\printfield{doi}
\newunit\newblock
\iftoggle{bbx:eprint}{
\usebibmacro{eprint}
}{}%
\newunit\newblock
\iffieldundef{doi}{
\usebibmacro{url+urldate}}
{}
}
\addbibresource{\jobname.bib}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@book{key,
author = {Author, A.},
year = {2001},
title = {Title},
publisher = {Publisher},
archivePrefix = {arXiv},
arxivId = {1607.07249},
doi = {10.1007/978-3-319-49004-5_22},
eprint = {1607.07249},
url = {http://link.springer.com/10.1007/978-3-319-49004-5{\_}22},
}
@book{keyx,
author = {Author, A.},
year = {2001},
title = {Title},
publisher = {Publisher},
archivePrefix = {arXiv},
arxivId = {1607.07249},
url = {http://link.springer.com/10.1007/978-3-319-49004-5{\_}22},
}
\end{filecontents*}
\begin{document}
\nocite{*}
\printbibliography
\end{document}