documentclass[a4paper]{article}
usepackage{amsmath}
usepackage{calc}

newcommand{mv}[1]{% metavariable
  mathit{#1}%
}
newcommand{sor}{% syntax or
  mathrel{makebox[widthof{$to$}]{$|$}}%
}
newcommand{sTO}{mathrel{texttt{=>}}}
newcommand{sto}{mathrel{texttt{->}}}
newcommand{slp}{mathopen{texttt{(}}}
newcommand{srp}{mathopen{texttt{)}}}
newcommand{slb}{mathopen{texttt{[}}}
newcommand{srb}{mathopen{texttt{]}}}
newcommand{slc}{mathpunct{texttt{,}}}

begin{document}

begin{figure}
centering

footnotesize % adapt to your page size
fbox{%
  $begin{aligned}
  mv{polytype} & to  mv{context} sTO mv{monotype}
  && \
                & sor mv{monotype}
  && \[2ex]
  mv{monotype} & to  mv{tyvar}
  && text{Type variable} \
                & sor mv{monotype}_1 sto mv{monotype}_2
  && text{Function type} \
                & sor slbmv{monotype}srb
  && text{List type} \
                & sor slpmv{monotype}_1slcdotsslcmv{monotype}_nsrp
                       quad (n=0text{ or }nge2)
  && text{Tuple type (incl. unit)} \
                & sor mv{tycon},mv{monotype}_1dotsmv{monotype}_n
                       quad (nge0)
  && text{Constructed type} \[2ex]
  context       & to slpmv{class}_1,mv{tyvar}_1slcdotsslcmv{class}_n,mv{tyvar}_nsrp
  end{aligned}$%
}

caption{Abstract syntax of Haskell: types}

end{figure}

end{document}