Cleveref messes up theoremstyles

Question

I use different theorem enviroments that should in theory all have the same style due to my code. They do as long as I don't load the cleveref package. If I load it, only corollary stays the same. How can I make cleveref to behave ( = to not disturb the styles) ?!

\documentclass{amsbook}
\usepackage{amsmath}
\usepackage{cleveref}
\theoremstyle{plain}
\newtheorem{corollary}{Corollary}\numberwithin{corollary}{chapter}
\newtheorem{example}[corollary]{Example}
\newtheorem{lemma}[corollary]{Lemma}
\begin{document}

\begin{corollary}
A corollary.
\end{corollary}

\begin{example}
An example.
\end{example}

\begin{lemma}
A lemma.
\end{lemma}

\end{document}

The messed up version looks like this:

And what I want is that all theorem environments look like the corollary. As I said, they do if I don't load cleveref.

Answer 1

In my opinion the amsbook class takes the wrong approach about theorems: it defines a slightly different structure from amsthm and fakes this package being loaded, so putting cleveref at stake, because this package does as if amsthm were actually loaded.

My solution is to forget about the setup made by the class and returning to amsthm, defining a new theorem style that emulates what amsbook does to begin with. A bit complicated, but it ensures cooperation with cleveref.

\documentclass{amsbook}

\usepackage{amsmath}

% nullify amsbook setup about theorems
\expandafter\let\csname [email protected]\endcsname\relax
\let\theoremstyle\relax
\let\newtheoremstyle\relax
\let\pushQED\relax
\let\popQED\relax
\let\qedhere\relax
\let\mathqed\relax
\let\openbox\relax
\let\proof\relax\let\endproof\relax
\usepackage{amsthm}
\usepackage{cleveref}

% make a new setup that emulates `amsbook`
\makeatletter
\renewcommand{\thm@space@setup}{% like amsbook does
  \thm@preskip=.5\baselineskip\@plus.2\baselineskip \@minus.2\baselineskip
  \thm@postskip=\thm@preskip
}
\newtheoremstyle{amsplain}
{\thm@preskip}
{\thm@postskip}
{\itshape}
{\parindent}
{\scshape}
{.}
{ }
{}
\makeatother

\theoremstyle{amsplain}
\newtheorem{corollary}{Corollary}[chapter]
\newtheorem{example}[corollary]{Example}
\newtheorem{lemma}[corollary]{Lemma}

\begin{document}

Some text before so that it shows indentation
and let's go on until the text breaks across lines
to show also the left margin.

\begin{corollary}
A corollary.
\end{corollary}

\begin{example}
An example.
\end{example}

\begin{lemma}
A lemma.
\end{lemma}

\end{document}

Answer 2

cleveref does a lot of sophisticated things with counters and references, using it before \newtheorem{example} etc. will not inform cleveref that example should use the corollary counter etc. (the same for lemma too then)

General rule: Load cleveref as the last package, even after hyperref!!!

\documentclass{amsbook}
\usepackage{amsmath}

\theoremstyle{plain}
\newtheorem{corollary}{Corollary}\numberwithin{corollary}{chapter}
\newtheorem{example}[corollary]{Example}
\newtheorem{lemma}[corollary]{Lemma}


\usepackage{cleveref}

\begin{document}

\begin{corollary}
A corollary.
\end{corollary}

\begin{example}
An example.
\end{example}

\begin{lemma}
A lemma.
\end{lemma}

\end{document}